In this order I like Beastmen, Lizardmen, Chaos Warriors, and Skaven. The few times that I have played 40k I am a generally "assaulty" player. I like the Beastmen because they have an appealing look as well as history, Lizardmen are pretty neat looking and would be a blast to paint, Chaos Warriors seem pretty "ownage" plus I like there horses, and Skaven are pretty much Skaven.