Joker.
Vader was a cool badass, but when you get down to it he's just a hitman. He is, fundamentally, understandable; "order in the galaxy by whatever means" really comes down to "I will make people do what I want them to do, by force if necessary". He can be reasoned with. He can be predicted.
Take the Joker. He is what is scary about chaos: he breaks down all of the structure that mankind builds up. The fear deep within us is simple: we inherently expect the universe, and by extension other men, to be rational. If I do X, then you will do Y. If I push an object, it moves. To have anything else happen undermines this most fundamental assumption. This is the fear that the Cthulhoid mythos plays off of, and it is this same fear that the Joker embodies.