Joker would be one of mine as well. He's just so unpredictable, enigmatic, and entertaining to watch. Not to mention his jokes go great with my dark sense of humor. My favorite though would be Scarecrow. I love manipulative and intelligent characters (whether they're villains or heroes), especially when they use fear to achieve their goals. Scarecrow is probably the epitome and poster child of a fear manipulator.
Scarecrow. I like the idea of using people's fear as a weapon. I also like It otherwise known as Bob Gray or Pennywise from the Steven King book. That creature also used fear. And so does Freddy from Nightmare on Elm St.