Personally I got to say Possessed; loved Marauders in WAR, Loved Argel Tal and the Gal Vorbak in First Heretic.
Torn between, Sorceror, Dark Apostle & War Smith. But I will have to go with Sorceror due to how iconic a class it is.
I would also recommend reading the Word Bearers' Omnibus (which contains the books: Dark Apostle, Dark Disciple and Dark Creed) that also has another Possessed marine called Burias who's the icon bearer of the Host. I'll leave it at that as to not give away any spoilers, because the story is pretty interesting. I would also say Possessed, as they are probably my favorite among the unique chaos classes. The ability to merge with a daemon, rip tanks apart with your bare claws, as well as slaughtering enemies effortlessly, is far more entertaining for me, then the other classes. Also, RIP Argel Tal.
Well you have your true possessed like Argel and the False Possessed like the Varak Jhal (I think they're called? been a while since I read Betrayer). True Possessed are immensely powerful and cool, False Possessed while equally cool are a little on the Mooky NPC side of things.
Sorcs, also theres is no current set number of classes that will appear for each race. So the topic name is kinda misleading.
Well there HAS to be khorne berzerkers,plague marines, and noise marines without question, because who doesn't like to Maim kill burn , drop the BASS and spread the touch of good ol grandfather nurgle?!
I pretty sure there was a set a classes for Chaos, the known classes were Raptor, Chosen, Havoc and Terminators. I know it's written somewhere... I remember reading it. Edit: now I remember, it was right here:
