Going to have to go with The Horned Rat, "the greatest trick the devil ever pulled is convincing the world he doesn't exist" Added to the complete and utter lack of interest in the well-being of his insane pack of followers, their incessant drug use, and the wacky ways they find to kill or change things just makes him that much more interesting to me.
Out of the big four, Tzeentch. Twisting the world around him to be what he wants it to be