Q:

Most common ways of remarking something is weird, strange, or unusual?

A very common remark in English is simply weird, or that's weird, to express that something is generally unexpected. What's the most common way of expressing this in French?
I don't want to just look up the translation for "weird" and come up with words like étrange or  bizarre, because that may not be the way this idea (of something being expected) is most commonly expressed in French, just as only is more often translated by ne...que than by seulement.
Although I seem to recall seeing c'est bizarre fairly often.

A:

C'est bizarre or c'est étrange are generally the two most used expressions to describe that something is weird. I wouldn't say that one of those is way more used than the other. For instance, to say :

The door is open ? That's weird.

You would usually say :

La porte est ouverte ? C'est bizarre.
La porte est ouverte ? C'est étrange.

Sometimes people will turn it in the idea of something "suspicious" : c'est louche. You can sometimes hear the slang version of this expression : c'est chelou.
You may also hear instead that "it is not normal" : ce n'est pas normal.

La porte est ouverte ? Ce n'est pas normal.

It does apply to the idea of something unusual. You may also hear c'est inhabituel (literally "it's unusual") or ce n'est pas commun ("this is not common").

