Yeah if we're going to answer honestly...
Tau, Eldar, Mechanicus
A better universe through technological superiority and justified smugness.
Also diplomacy (except for the Eldar, but they're just that superior).
I really love Eldar, Mechanicus and Kroot, however their hats just don't compare. I really like the whole idea with the Eldar, and just how far they fell, constantly being reminded of how usurped they are and how declining their entire species is. Their greatest craftworlds which to the lesser races are insane relics of immense power, are to them what were their basic merchant vessel. And they only have themselves to blame for this mess, and what caused it?
Boredom. The Eldar defeated death itself, and were killed by boredom. I love that
Mechanicus are amazing, they are hyper-rational robots, yet they managed to get across how it is possible to replace the flesh with machine and become a hyper-rational
human. It is an immense source of joy to see purity robit humans acting out the functions of humanity with the language of robits, I think the pinnacle of this is in
the first heretic where a mechanicus guy delivers some final protocols for his fallen servitors ;-;
Imperium is by far my favourite faction. So much promise, so much time spent in a glorious age - and it's all over. That age is gone and it will never come back, the end is coming from all sides without and within, the only reason all hasn't already collapsed is that the threats are so numerous and the imperium so vastly clusterfucked that no one threat is capable of fully destroying the other, leaving the horrendous bureaucratic galactic mess slogging it out in vast attrition warfare with the entire universe. At its heart is one God who is literally the lighthouse guiding all of mankind on the rocks, prolonging humanity's existence out of sheer stubbornness and arbitrary determination in the human condition. This God feeds on thousands of his people every day to live, and every day hears the prayers of trillions of worlds, all screaming out in pain and fear to a God who fought to be a Man. Every prayer a mockery of all he believed in and fought for, a never ending reminder of his failure to his progeny and project, twisting on his already shattered psyche as even his frame has deteriorated to only machine and bone. He can never wake from this nightmare, he can never turn back on his worshipers, he
must protect them even as he feels the weight of
all suffering, he didn't ask for this and he isn't even allowed to die; this is the grim dark future of the Imperium of Man circa 40,000.
But why no Dark Eldar?
Dark Eldar is what I am terrified of becoming, and constantly have to resist. Which makes me a normal Eldar
Dark Eldar are what happens when hope dies.
Dark Eldar know when they die they enter the hell they created, so they determined to not ever die. A whole colony of Dorian Grays stretched over aeons, only their portrait is not a painting, but the pain of their victims
soooooooo ...
If chaos is a humanity-creation, how does the eldar make a daemon-baby? Did their fucking not make it before but suddenly do so after?
Chaos is a creation of the necrontyr/Old One war in heaven
Humanity sustains it from the horus heresy onwards but that's just because they grew more psychically nutritious than eldar from that point on