Possessed and Raptors, although I've heard of success stories with both. I don't like the idea of Raptors being seen as second-rate tank poppers when there's an entire cult based on them. They deserve more dammit!
I also wish that the Chaos Bikers, Thousand Sons and Noise marines costed less points. I actually like Noise marines and the Thousand sons, but I simply think that the bikers cost too much. I don't actually like them that much.