It has to be the Night Lords for me: - they fight for themselves and follow their own path rather than doing what the Ruinous Powers tell them to do - they make use of lots of Raptors (and I love jump packs) - they can see in the dark - Aaron Dembski-Bowden wrote the best 40K novels outside of the Horus Heresy series about them - Joseph Conrad's Heart of Darkness is great - out of all the Traitor Legions they're the best dressed, the least mutated and the most cynical - in a Gothic grimdark IP they are the most Gothic grimdark space marines in it - their Primarch was basically the ultimate Batman, he's dead, and few of them care - they're basically pirates and everybody knows that pirates are cool! For the 8th! Ave Dominus Nox! I'm actually surprised to see the Iron Warriors are not more popular.
I suppose the issue with Iron Warriors is that once you take away their siege kit, they're essentially the Black Legion with a better paint job.
I've always loved the Death Guard because their "gift" makes them unique. I see the rest of them as just baddies in different colored armor.
If they allow me the change my paint scheme I will color my self a Thousand Son and become a sorcerer.
FOR THE XVII LEGION, FOR LORGAR, FOR THE DARK GODS OF CHAOS, IN THEIR NAME WE SHALL TOPPLE THE ROT THAT IS THE IMPERIUM!