Literally all of them.

Around the world?

Sure there's no objectivity to it, everyone has their own conception of culture which in many ways contradicts the next persons but my point is that everyone thinks their beliefs and customs and whatever are the best.

Sure thing lets do better than that.

It's absurd to reject supremacy in the broad way you define it

Not really, but lets pretend that white supremacy isn't a big part of the way modern society has been constructed.

How could you be a Christian, for example, if you supposed Christianity was no better than Atheism, Islam, Hinduism, Satanism or whatever else?

By not thinking it was better than Atheism, Islam, Hinduism, Satanism or whatever else.

Obviously everyone must think that their beliefs are the best because otherwise why the hell would you believe them?

You can't really conflate beliefs and culture. They're not the same thing. Like I'm culturally a Kiwi but I also am highly critical of the culture here.