Papa Nurgle or Tzeentch, those two are the best But I prefer to be one of Thousand Sons than being a smelly, decaying Plague Marine (yes, I know that TS are cursed, have no bodies, they are just souls trapped in powerarmors, but this is badass!)
Starchild!!!!!!! Malal!!!!! Well considering my first two choices aren't up there, that just leaves the only other choice, Tzeentch. Chaos undecided, I mean undivided is just for those who haven't chosen yet.
The offerings of lesser gods cannot compare to the gifts grandfather bestows, all pain becomes a mere drop in an ocean and death holds no fear. The rot is a bouquet for the blessed. tbh i could dig any chaos god but id love to have the choice. One of the things that peeved me about Warhammer online was their attitude to the chaos gods in so much as they wanted a kiddy friendly game and only tzeench could limbo under that bar. I say if you want to do warhammer right you cant have that attitude.
This isn't Warhammer online, this is Warhammer 40k, which is different in it's own way. This game isn't going to be kiddy friendly, especially when games like Dawn of War include a lot of bloodshed and books like Fulgrim describe a lot of gore.