user8157: Many suttas (SN 6.11; SN 21.11; MN 53; AN 11.10; DN 27) contain the following phase:

‘The aristocrat is best of those people ‘khattiyo seṭṭho janetasmiṃ,
who take clan as the standard. ye gottapaṭisārino;
But one accomplished in knowledge and conduct vijjācaraṇasampanno,
is best of gods and humans.’ so seṭṭho devamānuse’ti.

Why is the aristocrat best of those people who take clan as the standard? 

user254: I assume it means, "Among or according to the those who take the clan as standard -- i.e. according to those who judge according to the clan in which a person is born -- the aristocrat is best."
That was a conventional/social belief at the time -- i.e. that there were clans or castes. And according to that maybe the aristocrat was best by definition -- the sutta says Khattiyo, that's presumably "Kshatriyas" mentioned in the Varna (Hinduism) article.
Compare with e.g. the Dhammapada, "It is not by birth that one is a 'Brahmin', a holy man" -- see also DN 4.

user471: Interestingly, in AN 5.179, it lists aristocrats before other castes (vaṇṇa or varna):

among aristocrats, brahmins, merchants,
Khattiye brāhmaṇe vesse,
workers, or outcastes and scavengers —
sudde caṇḍālapukkuse.

In the suttas you quoted, it says "the aristocrat is best of those people, who take clan as the standard". The word for "clan" here is gotta or gotra, which is different from caste (which has two types of concepts - vanna or varna, and jāti). The caste system from the Hindu scriptures is varna.
Gotta or gotra is an unbroken male lineage from a common male ancestor. Two persons from the same gotta or gotra cannot get married.
From a caste perspective, Hinduism takes brahmana to be the highest caste, followed by khattiya or kshatriya.
However, from a clan perspective, the suttas take the khattiya or kshatriya i.e. aristocrat, to be the best of people (jana).
Perhaps the reason for this is that kings are always khattiya or kshatriya, and their lineage is always an unbroken male line from the first king of that dynasty. Since a king is the best of people in any country, and his clan (in terms of ancestral lineage) is always aristocrat, so aristocrats are presumably the best of people in a country from the perspective of clans (gotta).

user14690: Taken in the context of a conversation between the Buddha and a Brahman in the Ambaṭṭhasutta (DN 3), it is quite clear how this verse is supposed to be interpreted. 
Given the hierarchy of caste, the Buddha argues, brahmans are indeed inferior to aristocrats (i.e. rulers and nobles). He depicts brahmans as subservient to the aristocratic class, and indicates that the aristocratic class is more exclusive -- an aristocrat could become a brahman, but a brahman could never become an aristocrat. 
Thus, this verse should be taken as an argument against brahmanical superiority. Rather -- and this is something repeated in several other places -- a mendicant has worth not on account of their birth, but their sound ethics, great wisdom, and role as a teacher (of dhamma). This is a standard argument against caste hierarchy, which is depicted as illogical given how brahmans are subordinated to the nobility in practice. Instead, the Pali canon places the Buddha and the Buddha alone at the top of a newly configured hierarchy (ostensibly based on ethics and wisdom alone, but sometimes on the basis of Gotama's godlike preeminence among all beings).

8157: <p>Many suttas (SN 6.11; SN 21.11; MN 53; AN 11.10; DN 27) contain the following phase:</p>

<blockquote>
  <p>‘The aristocrat is best of those people ‘khattiyo seṭṭho janetasmiṃ,</p>
  
  <p>who take clan as the standard. ye gottapaṭisārino;</p>
  
  <p>But one accomplished in knowledge and conduct vijjācaraṇasampanno,</p>
  
  <p>is best of gods and humans.’ so seṭṭho devamānuse’ti.</p>
</blockquote>

<p>Why is the aristocrat best of those people who take clan as the standard? </p>
 254: <p>I assume it means, "Among or according to the those who take the clan as standard -- i.e. according to those who judge according to the clan in which a person is born -- the aristocrat is best."</p>

<p>That was a conventional/social belief at the time -- i.e. that there were clans or castes. And according to that maybe the aristocrat was best by definition -- the sutta says <a href="https://suttacentral.net/define/khattiya" rel="nofollow noreferrer">Khattiyo</a>, that's presumably "Kshatriyas" mentioned in the <a href="https://en.wikipedia.org/wiki/Varna_(Hinduism)" rel="nofollow noreferrer">Varna (Hinduism)</a> article.</p>

<p>Compare with e.g. the Dhammapada, "It is not by birth that one is a 'Brahmin', a holy man" -- see also <a href="https://suttacentral.net/dn4/en/sujato" rel="nofollow noreferrer">DN 4</a>.</p>
 471: <p>Interestingly, in <a href="https://suttacentral.net/an5.179/en/sujato" rel="nofollow noreferrer">AN 5.179</a>, it lists aristocrats before other castes (<em>vaṇṇa</em> or <em>varna</em>):</p>

<blockquote>
  <p>among aristocrats, brahmins, merchants,<br>
  <em>Khattiye brāhmaṇe vesse,</em></p>
  
  <p>workers, or outcastes and scavengers —<br>
  <em>sudde caṇḍālapukkuse.</em></p>
</blockquote>

<p>In the suttas you quoted, it says "<em>the aristocrat is best of those people, who take clan as the standard</em>". The word for "clan" here is <a href="https://en.wikipedia.org/wiki/Gotra" rel="nofollow noreferrer"><em>gotta</em> or <em>gotra</em></a>, which is different from <a href="https://en.wikipedia.org/wiki/Caste_system_in_India" rel="nofollow noreferrer">caste</a> (which has two types of concepts - <a href="https://en.wikipedia.org/wiki/Varna_(Hinduism)" rel="nofollow noreferrer"><em>vanna</em> or <em>varna</em></a>, and <a href="https://en.wikipedia.org/wiki/J%C4%81ti" rel="nofollow noreferrer"><em>jāti</em></a>). The caste system from the Hindu scriptures is <em>varna</em>.</p>

<p><em>Gotta</em> or <em>gotra</em> is an unbroken male lineage from a common male ancestor. Two persons from the same <em>gotta</em> or <em>gotra</em> cannot get married.</p>

<p>From a caste perspective, Hinduism takes <em>brahmana</em> to be the highest caste, followed by <em>khattiya</em> or <em>kshatriya</em>.</p>

<p>However, from a clan perspective, the suttas take the <em>khattiya</em> or <em>kshatriya</em> i.e. aristocrat, to be the best of people (<a href="https://www.wisdomlib.org/definition/jana#pali" rel="nofollow noreferrer"><em>jana</em></a>).</p>

<p>Perhaps the reason for this is that kings are always <em>khattiya</em> or <em>kshatriya</em>, and their lineage is always an unbroken male line from the first king of that dynasty. Since a king is the best of people in any country, and his clan (in terms of ancestral lineage) is always aristocrat, so aristocrats are presumably the best of people in a country from the perspective of clans (<em>gotta</em>).</p>
 14690: <p>Taken in the context of a conversation between the Buddha and a Brahman in the <a href="https://suttacentral.net/dn3/en/sujato#dn3:1.23.23" rel="nofollow noreferrer">Ambaṭṭhasutta (DN 3)</a>, it is quite clear how this verse is supposed to be interpreted. </p>

<p>Given the hierarchy of caste, the Buddha argues, brahmans are indeed inferior to aristocrats (i.e. rulers and nobles). He depicts brahmans as subservient to the aristocratic class, and indicates that the aristocratic class is more exclusive -- an aristocrat could become a brahman, but a brahman could never become an aristocrat. </p>

<p>Thus, this verse should be taken as an argument against brahmanical superiority. Rather -- and this is something repeated in several other places -- a mendicant has worth not on account of their birth, but their sound ethics, great wisdom, and role as a teacher (of <em>dhamma</em>). This is a standard argument against caste hierarchy, which is depicted as illogical given how brahmans are subordinated to the nobility in practice. Instead, the Pali canon places the Buddha and the Buddha alone at the top of a newly configured hierarchy (ostensibly based on ethics and wisdom alone, but sometimes on the basis of Gotama's godlike preeminence among all beings).</p>
