So… T’au are the most based faction in the universe, right?

I would say craftworld eldar (also called Asuryani). They are unpleasant and arrogant, but they are basically dying race whose future include being dead and getting souls eaten and presumably eternally tormented in hell, so being grumpy as fuck is understandable.

Their society though is basically communist, they have no classes, no money, complete gender equality, live in postscarcity economy, their entire army is basically armed citizens plus some dedicated specialists and even their labour division is very fluid since they change occupation every few years or decades and most eldar can eventually be anyone they want. They even have juche necromancy where their dead can still participate in society in various forms from counseling to personal operation of various constructs.

Their only problem is that they usually consider everyone else as threat and sometimes perpetrate horrible war crimes for seemingly no reason whatsoever because they rely on future sight and prophecies to fight off various threats.

There are also exodite eldar who are basically anprims riding huge dinosaurs and just minding their own business which gets as good as warhammer can go. Pity they only ever appear in the lore as victims of genocide perpetrated by someone else.