What sort of force do you prefer? Frothing assault loonies? Big Plague lumps that can take a beating, Magic users?
You can go either of 4 ways with a cult army; World Eater Berzerkers, Slaaneshy Noise Marines, Nurgle Plague marines or the Tzeentchian Rubric marines)
Bezerkers: Not really much to be said here, rock hard melee specialist that excel in close combat
Noise Marines: High initiative users of sound based weapons, very shooty
Plague Marines: As I said before, these guys can take some punishment, tougher than average for MEQ's and having FNP is niiiice
Rubric Marines: Otherwise known as Thousand Sons. These guys are the bane of Space marines and all MEQs, They have AP3 bolters and are more resilient than average with a 4+ Inv save.
That's just the quickest over view of the chaos Cults, you can of course combine them all to make a super killy army of death!