user-1: Buddhism begins with The Truth of Suffering. That seems like the best point to get someone's attention, and is basically indisputable. The Truth of Impermanence is fairly easy to accept, because we see things change, and everyone dies. But the Truth of Not-Self - no separate, permanent self - is the major departure from Hinduism, and all other religions, as best I know. Realization of this truth amounts to nonduality.
If someone can realize Anatman then is this not basically the whole teaching? With the large number of Self-Inquiry groups and websites devoted to nonduality and the many interviews and books about it available, it seems like a more direct path to realization, and more accessible than Buddhism (in its many forms). Is it?
EDIT: I seem to be drawing fire for my phrase "the whole teaching". OK, disregard that but the point is: if you "get" Anatman, doesn't everything else have to fall in to place over time? With that one realization, you can realize everything, and without it, you will never make it all the way. Is that not so?

user7290: It is said that wisdom (about emptiness) and compassion are the most important concepts in Buddhism. But then agian, how can concepts be the most important thing in Buddhism? 
If one focuses too much on the anatta-side, one risks getting into nihilism. So the emptiness/no-self doctrine is important, but if one does not supplement it with compassin, there is no use. 

user5513: The most important or the final concept in Buddhism is realizing "Four Noble Truths".  
All the other concepts are within it. and should not separately consider. 
According to the first sermon "Dhammacakkappavattana Sutta", the base point of understanding is realizing "Anichcha".

That is what the Blessed One said. Gratified, the group of five monks
  delighted at his words. And while this explanation was being given,
  there arose to Ven. Kondañña the dustless, stainless Dhamma eye:
  Whatever is subject to origination is all subject to cessation.


user295: Nirvana is the central concept of Buddhism, there 37 factors which cover the teachings leading to Nirvana. When you practice this as get to Nirvana you see the 3 marks of existence of which one is anattā

user-1: Practically speaking, I think the most important concept in Buddhism is that through our own efforts, we are all capable of enlightenment.  Teachings of any sort - be they on non-self, suffering, or impermanence - are ultimately meaningless unless we are capable of direct realization ourselves.  Without that possibility, the entirety of Buddhism is just an empty philosophy (no pun intended!).  Believe it or not, the idea that individual agency could result in liberation was quite scandalous at the time of the Buddha (e.g. the Brahamajala Sutta).  Like most examples of genius, it's something we take for granted now.
I think it's a bit presumptuous to say that impermanence and suffering are obvious.  They are just a subtle as anatman and, in fact, cannot be understood in full unless the others two marks of existence are also seen directly.  Ultimately what is most important is our potential to make this realization for ourselves.     

user471: I think a very important concept in Buddhism is the middle way. From the perspective of practice, it refers to the middle path between indulgence and asceticism. From the perspective of philosophy, it refers to the middle way between eternalism and annihilationism. This theme keeps reappearing everywhere in Buddhism and seems to be unique to it. This is discussed in this answer.
Anatta is the concept where there is no absolute eternal permanent self, that is behind all phenomena. At the same time, there is a self that is not annihilated completely at death. The self is not standalone and emerges out of the interworking between form, sensation, perception, mental formations and consciousness. How the perception of self is formed is covered in dependent origination.
Anicca too has a connection to this middle way. The universe is neither eternal and absolute, nor is it an illusion. It is real, but it is always changing. However, the universe is empty as in devoid of a permanent self.
Another important middle way concept in Buddhism is that the person that is reborn is neither exactly the same nor totally unrelated from the person in the previous life. This is discussed in this answer.
In physics too, similar ideas appear in quantum mechanics, with the wave-particle duality as an example.
In Advaita Vedanta, Atman (the Self) is permanent, standalone and not different from Brahman, the ultimate eternal reality. The Atman is also considered the Eternal Witness, which extends to the idea of Eternal Consciousness, that every being has the same "I" which is the Atman, which is ultimately Brahman. The Self Inquiry method of Sri Ramana Maharshi asks "who am I?" and seeks to lead the spiritual seeker towards Self Realization, that "I" am not this temporary person (the little "I"), but rather the Eternal Consciousness (the big "I"), the Atman, who is Brahman.
This is quite incompatible with Buddhism, because the Buddha refuted not only a permanent self (in this answer), but also the idea of Eternal Consciousness. In this sutta, the Buddha rebuked Sati the monk, for holding the view of Eternal Consciousness (related to Atman), and corrects him by teaching the concept of eye-consciousness, ear-consciousness etc. According to this, consciousness is not an Eternal Witness, but rather, arises with sensation and perception. According to this sutta, the five aggregates are empty as in, devoid of a self.
Hence Advaita Vedanta is quite different and incompatible with Buddhism. However, according to this question, what is common or similar between the two is the fact that both Nirvana and Brahman are eternal, unborn and undying.

user254: If or given that Buddhism "begins with The Truth of Suffering", then I'd guess that "cessation" (of suffering, e.g. the Third Noble Truth) might be fairly high on the list of important concepts.

From a Theravada perspective maybe Saṅkhāra is important. Arguments for its being important can be inferred from that Wikipedia article, i.e.:

The Buddha's exclamation about the "housebuilder" when he became enlightened
The Buddha's last words

Anatman might be a special case of Saṅkhāra, i.e. if you understand Saṅkhāra then you understand Anatman (because the conventional "self" is an example of a put-together fabrication).
Understanding Saṅkhāra presumably results in understanding Anicca too.

From a non-Theravada point of view I recommend this answer:

It suggests there isn't a single most important concept (which is important in 'understanding Buddhism'; see also "Buddhism is a highly individualized practice" from newbold's answer; and see also the Buddhist Paths to liberation being plural)
I also like that answer for his description of the Tathagata-Garba.


Alternatively maybe compassion is the most important concept, the 'sine qua non' of Buddhism (i.e. the characteristic without which Buddhism wouldn't exist because the Buddha wouldn't have taught his doctrine), if I'm right in assuming that compassion is what motivates Bodhisattvas.

And of course Bodhi is an important concept, as a goal and as inherent in the name "Buddhism".

user-1: Anatta is one of the 3 marks of existence. The others being; anicca and dukkha.
The whole teaching is the "most" important. One cannot just take out a piece of the teaching. It does not work like that.
The Noble Eightfold Path is a complete teaching that needs nothing to be added or removed from it.
Sila, Samadhi, Panna. All groups are needed in order to win Nibbana.

user793: The subtlety here is that understanding of this truth is not realization of this truth.  I might have an experience of ego-shattering transcendence and yet, the very (next) moment, act selfishly out of long-worn old habits.  The practice of realization is one of purification and ceasing to establish new roots for future suffering.
In the Zen tradition we often chant/study the Heart sutra, which sums up the fruition of practice as "gate gate paragate parasamgate, bodhi svaha".  Loosely, "gone, gone, gone beyond, gone completely beyond, awake, so be it."  It's not enough to understand anatta, I must go beyond even understanding not-I and find myself realized in living my life.  I have to go beyond going and find a way to be present by ceasing to imagine that I am (not).  Which is all just a fancy way of saying this is a real practice to be lived and not merely a philosophy to be understood.

user7716: In order to understand the underlying cause of ignorance, or the human propensity towards false notions of self, anatman is essentially important. In order to fully fathom interdependency, it is essentially important. When the Buddha did not accept, or rejected, the notion of selfhood, the kind of self that was being rejected was characterized by three features: a self that is unitary, that is permanent, that is independent. And so then the question followed, what is the nature of self ? how can we understand the existence and nature of self ?
In one of the sutras, it states that just as the notion of a horse and carriage and chariot arises  on the basis of the collection and aggregation of its parts,  in the same manner, from the collection of various mental and physical elements, the concept of the person or the individual arises. So although in reality self is a phenomenon that is contingent upon the existence and  reality of the physical and mental aggregates, when we perceive our own self, when the concept, the notion of self arises in us,  in our naïve understanding of our nature, it tends to appear to us as if it is somehow self sufficient, as some kind of self governing reality.
This notion of self as a self sufficient substantial reality is false, and not only is it false, but the grasping at this notion of self is then a form of distortion. In this grasping the self is elevated to an independent and self sufficient status.  Also there is a risk when we focus on, when we reference as reality, the continuity of the self (when I was young, when I was middle aged etc), in that if one focuses too much on the continuity of the self there is the danger of holding on to this notion of a permanent self. In the Buddhist vernacular, this distorted perception is referred to as ignorance... [paraphrased/extracted from a 2009 talk by HH The Dalai Lama]

user2207: Buddhism has many core principles which work as complete paths.  However, like other projects, it helps to use a variety of techniques to cover all bases.
The unifying principle in Buddhism is letting go of clinging to reduce suffering.  Looking at some core Buddhist principles through this lens, you will see that each address clinging in a different -- yet compatible -- way.

Realizing Emptiness undermines our belief in the ultimate existence of the things we cling to.
Realizing No Self undermines our belief in the one who clings and is a specific instance of emptiness.
Realizing Suffering undermines our desire to cling in the first place by noting the dissatisfaction of all we cling to.
Realizing Impermanence undermines our desire to cling in the first place by noting that what we cling to will change (or vanish) and hence hurt us.

So the question here isn't whether any concept -- including Anatman -- is sufficient in itself.  I believe the answer to that question is yes.  Rather the questions are...

Is your realization of any of the concepts complete enough that you
don't need the others?
Are you facing any particular challenges that may make some concepts
particularly beneficial for you?


user43: No, anatta is not the key point of Buddhism, and Buddhism is not nondualism.
That would be too easy ;)
Shunyata, the Mahayana extrapolation of anatta principle, is getting closer.
If I had to pick one point to explain it all, I would say TATHATA.

user7233: 
If someone can realize Anatman then is this not basically the whole teaching?

It's certainly a great step forward on the path, but it is not the whole teaching.
This is the whole teaching.
That was a joke! But seriously, there is no truly objective answer to your question. Anattā is just one slice of one of the many pies of Buddhism. As a "mark of existence" it holds equal importance alongside the other two, dukkhā and aniccā.

If someone can realize Anatman then is this not basically the whole teaching?

What about saṃsāra, kamma, the Noble Eightfold Path, the Middle Way, the Four Immeasurables, dependent origination, emptiness, and The Three Jewels? And a bunch of other stuff?
I too crave simplicity in Buddhism, but the more you look around, the deeper and broader the teachings become. There are 43 volumes in the Pāli Canon. The teachings are vast.

If someone can realize Anatman then is this not basically the whole teaching?

It could be your whole teaching. Buddhism is a highly individualized practice. Our paths may run parallel, or at times criss-cross, or at other times go in entirely different directions: but in the end, we can all reach our own way to the end of suffering, and each way may be truly unique.

-1: <p>Buddhism begins with The Truth of Suffering. That seems like the best point to get someone's attention, and is basically indisputable. The Truth of Impermanence is fairly easy to accept, because we see things change, and everyone dies. But the Truth of Not-Self - no separate, permanent self - is the major departure from Hinduism, and all other religions, as best I know. Realization of this truth amounts to nonduality.</p>

<p>If someone can realize Anatman then is this not basically the whole teaching? With the large number of Self-Inquiry groups and websites devoted to nonduality and the many interviews and books about it available, it seems like a more direct path to realization, and more accessible than Buddhism (in its many forms). Is it?</p>

<p><strong>EDIT:</strong> I seem to be drawing fire for my phrase "the whole teaching". OK, disregard that but the point is: if you "get" Anatman, doesn't everything else have to fall in to place over time? With that one realization, you can realize everything, and without it, you will never make it all the way. Is that not so?</p>
 7290: <p>It is said that wisdom (about emptiness) and compassion are the most important concepts in Buddhism. But then agian, how can concepts be the most important thing in Buddhism? </p>

<p>If one focuses too much on the anatta-side, one risks getting into nihilism. So the emptiness/no-self doctrine is important, but if one does not supplement it with compassin, there is no use. </p>
 5513: <p>The most important or the final concept in Buddhism is realizing "<strong>Four Noble Truths</strong>".  </p>

<p>All the other concepts are within it. and should not separately consider. </p>

<p>According to the <strong>first sermon</strong> "<a href="http://www.accesstoinsight.org/tipitaka/sn/sn56/sn56.011.than.html" rel="nofollow">Dhammacakkappavattana Sutta</a>", the base point of understanding is realizing "Anichcha".</p>

<blockquote>
  <p>That is what the Blessed One said. Gratified, the group of five monks
  delighted at his words. And while this explanation was being given,
  there arose to Ven. Kondañña the dustless, stainless <strong>Dhamma eye</strong>:
  <strong><em>Whatever is subject to origination is all subject to cessation</em></strong>.</p>
</blockquote>
 295: <p><a href="https://en.wikipedia.org/wiki/Nirvana" rel="nofollow">Nirvana</a> is the central concept of <a href="https://en.wikipedia.org/wiki/Buddhism" rel="nofollow">Buddhism</a>, there <a href="https://en.wikipedia.org/wiki/Bodhipakkhiy%C4%81dhamm%C4%81" rel="nofollow">37 factors</a> which cover the teachings leading to Nirvana. When you practice this as get to Nirvana you see the <a href="https://en.wikipedia.org/wiki/Three_marks_of_existence" rel="nofollow">3 marks of existence</a> of which one is anattā</p>
 -1: <p>Practically speaking, I think the most important concept in Buddhism is that through our own efforts, we are all capable of enlightenment.  Teachings of any sort - be they on non-self, suffering, or impermanence - are ultimately meaningless unless we are capable of direct realization ourselves.  Without that possibility, the entirety of Buddhism is just an empty philosophy (no pun intended!).  Believe it or not, the idea that individual agency could result in liberation was quite scandalous at the time of the Buddha (e.g. the Brahamajala Sutta).  Like most examples of genius, it's something we take for granted now.</p>

<p>I think it's a bit presumptuous to say that impermanence and suffering are obvious.  They are just a subtle as anatman and, in fact, cannot be understood in full unless the others two marks of existence are also seen directly.  Ultimately what is most important is our potential to make this realization for ourselves.     </p>
 471: <p>I think a very important concept in Buddhism is the middle way. From the perspective of practice, it refers to the middle path between indulgence and asceticism. From the perspective of philosophy, it refers to the middle way between eternalism and annihilationism. This theme keeps reappearing everywhere in Buddhism and seems to be unique to it. This is discussed in <a href="https://buddhism.stackexchange.com/a/8164/471">this answer</a>.</p>

<p>Anatta is the concept where there is no absolute eternal permanent self, that is behind all phenomena. At the same time, there is a self that is not annihilated completely at death. The self is not standalone and emerges out of the interworking between form, sensation, perception, mental formations and consciousness. How the perception of self is formed is covered in <a href="https://buddhism.stackexchange.com/q/48/471">dependent origination</a>.</p>

<p>Anicca too has a connection to this middle way. The universe is neither eternal and absolute, nor is it an illusion. It is real, but it is always changing. However, the universe is empty as in devoid of a permanent self.</p>

<p>Another important middle way concept in Buddhism is that the person that is reborn is neither exactly the same nor totally unrelated from the person in the previous life. This is discussed in <a href="https://buddhism.stackexchange.com/a/12076/471">this answer</a>.</p>

<p>In physics too, similar ideas appear in quantum mechanics, with the <a href="https://en.wikipedia.org/wiki/Wave%E2%80%93particle_duality" rel="nofollow noreferrer">wave-particle duality</a> as an example.</p>

<p>In Advaita Vedanta, Atman (the Self) is permanent, standalone and not different from Brahman, the ultimate eternal reality. The Atman is also considered the Eternal Witness, which extends to the idea of Eternal Consciousness, that every being has the same "I" which is the Atman, which is ultimately Brahman. The Self Inquiry method of Sri Ramana Maharshi asks "who am I?" and seeks to lead the spiritual seeker towards Self Realization, that "I" am not this temporary person (the little "I"), but rather the Eternal Consciousness (the big "I"), the Atman, who is Brahman.</p>

<p>This is quite incompatible with Buddhism, because the Buddha refuted not only a permanent self (in <a href="https://buddhism.stackexchange.com/a/7697/471">this answer</a>), but also the idea of Eternal Consciousness. In <a href="http://www.accesstoinsight.org/tipitaka/mn/mn.038.than.html" rel="nofollow noreferrer">this sutta</a>, the Buddha rebuked Sati the monk, for holding the view of Eternal Consciousness (related to Atman), and corrects him by teaching the concept of eye-consciousness, ear-consciousness etc. According to this, consciousness is not an Eternal Witness, but rather, arises with sensation and perception. According to <a href="http://www.accesstoinsight.org/tipitaka/sn/sn35/sn35.085.than.html" rel="nofollow noreferrer">this sutta</a>, the five aggregates are empty as in, devoid of a self.</p>

<p>Hence Advaita Vedanta is quite different and incompatible with Buddhism. However, according to <a href="https://buddhism.stackexchange.com/q/5762/471">this question</a>, what is common or similar between the two is the fact that both Nirvana and Brahman are eternal, unborn and undying.</p>
 254: <p>If or given that Buddhism "<em>begins</em> with The Truth of Suffering", then I'd guess that "<em>cessation</em>" (of suffering, e.g. the Third Noble Truth) might be fairly high on the list of important concepts.</p>

<hr>

<p>From a Theravada perspective maybe <a href="https://en.wikipedia.org/wiki/Sa%E1%B9%85kh%C4%81ra" rel="nofollow noreferrer">Saṅkhāra</a> is important. Arguments for its being important can be inferred from that Wikipedia article, i.e.:</p>

<ul>
<li>The Buddha's exclamation about the "housebuilder" when he became enlightened</li>
<li>The Buddha's last words</li>
</ul>

<p>Anatman might be a special case of Saṅkhāra, i.e. if you understand Saṅkhāra then you understand Anatman (because the conventional "self" is an example of a put-together fabrication).</p>

<p>Understanding Saṅkhāra presumably results in understanding Anicca too.</p>

<hr>

<p>From a non-Theravada point of view I recommend <a href="https://buddhism.stackexchange.com/a/7275/254">this answer</a>:</p>

<ul>
<li><p>It suggests there isn't a <em>single</em> most important concept (which is important in 'understanding Buddhism'; see also "Buddhism is a highly individualized practice" from <a href="https://buddhism.stackexchange.com/a/12513/254">newbold's answer</a>; and see also the <a href="https://en.wikipedia.org/wiki/Buddhist_Paths_to_liberation" rel="nofollow noreferrer">Buddhist Paths to liberation</a> being plural)</p></li>
<li><p>I also like that answer for his description of the Tathagata-Garba.</p></li>
</ul>

<hr>

<p>Alternatively maybe compassion is the most important concept, the 'sine qua non' of Buddhism (i.e. the characteristic without which Buddhism wouldn't exist because the Buddha wouldn't have taught his doctrine), if I'm right in assuming that compassion is what motivates Bodhisattvas.</p>

<hr>

<p>And of course <a href="https://en.wikipedia.org/wiki/Bodhi" rel="nofollow noreferrer">Bodhi</a> is an important concept, as a goal and as inherent in the name "Buddhism".</p>
 -1: <p>Anatta is one of the 3 marks of existence. The others being; <em>anicca and dukkha</em>.</p>

<p>The whole teaching is the "most" important. One cannot just take out a piece of the teaching. It does not work like that.</p>

<p>The Noble Eightfold Path is a complete teaching that needs nothing to be <em>added or removed</em> from it.</p>

<p><em>Sila, Samadhi, Panna</em>. All groups are needed in order to win Nibbana.</p>
 793: <p>The subtlety here is that understanding of this truth is not realization of this truth.  I might have an experience of ego-shattering transcendence and yet, the very (next) moment, act selfishly out of long-worn old habits.  The practice of realization is one of purification and ceasing to establish new roots for future suffering.</p>

<p>In the Zen tradition we often chant/study the Heart sutra, which sums up the fruition of practice as "gate gate paragate parasamgate, bodhi svaha".  Loosely, "gone, gone, gone beyond, gone completely beyond, awake, so be it."  It's not enough to understand anatta, I must go beyond even understanding not-I and find myself realized in living my life.  I have to go beyond going and find a way to be present by ceasing to imagine that I am (not).  Which is all just a fancy way of saying this is a real practice to be lived and not merely a philosophy to be understood.</p>
 7716: <p>In order to understand the underlying cause of ignorance, or the human propensity towards false notions of self, anatman is essentially important. In order to fully fathom interdependency, it is essentially important. When the Buddha did not accept, or rejected, the notion of selfhood, the kind of self that was being rejected was characterized by three features: a self that is unitary, that is permanent, that is independent. And so then the question followed, what is the nature of self ? how can we understand the existence and nature of self ?</p>

<p>In one of the sutras, it states that just as the notion of a horse and carriage and chariot arises  on the basis of the collection and aggregation of its parts,  in the same manner, from the collection of various mental and physical elements, the concept of the person or the individual arises. So although in reality self is a phenomenon that is contingent upon the existence and  reality of the physical and mental aggregates, when we perceive our own self, when the concept, the notion of self arises in us,  in our naïve understanding of our nature, it tends to appear to us as if it is somehow self sufficient, as some kind of self governing reality.</p>

<p>This notion of self as a self sufficient substantial reality is false, and not only is it false, but the grasping at this notion of self is then a form of distortion. In this grasping the self is elevated to an independent and self sufficient status.  Also there is a risk when we focus on, when we reference as reality, the continuity of the self (when I was young, when I was middle aged etc), in that if one focuses too much on the continuity of the self there is the danger of holding on to this notion of a permanent self. In the Buddhist vernacular, this distorted perception is referred to as ignorance... [paraphrased/extracted from a 2009 talk by HH The Dalai Lama]</p>
 2207: <p>Buddhism has many core principles which work as complete paths.  However, like other projects, it helps to use a variety of techniques to cover all bases.</p>

<p>The unifying principle in Buddhism is letting go of clinging to reduce suffering.  Looking at some core Buddhist principles through this lens, you will see that each address clinging in a different -- yet compatible -- way.</p>

<ul>
<li>Realizing Emptiness undermines our belief in the ultimate existence of the things we cling to.</li>
<li>Realizing No Self undermines our belief in the one who clings and is a specific instance of emptiness.</li>
<li>Realizing Suffering undermines our desire to cling in the first place by noting the dissatisfaction of all we cling to.</li>
<li>Realizing Impermanence undermines our desire to cling in the first place by noting that what we cling to will change (or vanish) and hence hurt us.</li>
</ul>

<p>So the question here isn't whether any concept -- including Anatman -- is sufficient in itself.  I believe the answer to that question is yes.  Rather the questions are...</p>

<ul>
<li>Is your realization of any of the concepts complete enough that you
don't need the others?</li>
<li>Are you facing any particular challenges that may make some concepts
particularly beneficial for you?</li>
</ul>
 43: <p>No, anatta is not the key point of Buddhism, and Buddhism is not nondualism.<br>
That would be too easy ;)</p>

<p>Shunyata, the Mahayana extrapolation of anatta principle, is getting closer.</p>

<p>If I had to pick one point to explain it all, I would say TATHATA.</p>
 7233: <blockquote>
  <p>If someone can realize Anatman then is this not basically the whole teaching?</p>
</blockquote>

<p>It's certainly a great step forward on the path, but it is not the whole teaching.</p>

<p><strong><a href="https://i.stack.imgur.com/SNbxX.png" rel="nofollow noreferrer">This is the whole teaching.</a></strong></p>

<p><em>That was a joke!</em> But seriously, there is no truly objective answer to your question. Anattā is just one slice of one of the many pies of Buddhism. As a "mark of existence" it holds equal importance alongside the other two, dukkhā and aniccā.</p>

<blockquote>
  <p>If someone can realize Anatman then is this not basically the whole teaching?</p>
</blockquote>

<p>What about saṃsāra, kamma, the Noble Eightfold Path, the Middle Way, the Four Immeasurables, dependent origination, emptiness, and The Three Jewels? And a bunch of other stuff?</p>

<p>I too crave simplicity in Buddhism, but the more you look around, the deeper and broader the teachings become. There are 43 volumes in the Pāli Canon. The teachings are <em>vast</em>.</p>

<blockquote>
  <p>If someone can realize Anatman then is this not basically the whole teaching?</p>
</blockquote>

<p>It could be <em>your</em> whole teaching. Buddhism is a highly individualized practice. Our paths may run parallel, or at times criss-cross, or at other times go in entirely different directions: but in the end, we can all reach our own way to the end of suffering, and each way may be truly unique.</p>
