Well, there are too many in real life to go into. So I'm gonna go with Supernatural, as usual. It's kind of a toss up between Azazel and Lilith. I mean, in In the Beginning, Dean TOLD Azazel he was going to kill him in the future and Azazel still didn't kill him. Just toyed with him a bit and went off to have more fun elsewhere (and then Dean killed him. Yeah Dean:). And Lilith....well, at least she tried, you know? She shredded Dean with her hell hounds and tortured him for forty years, and she tried to kill Sam.....but they are still so gonna get her. So maybe I'll just have to nominate them both. Oh, and Castiel and Uriel too, 'cause they threatened both Sam AND Dean
for no reason, and as we've seen before, pretty much no one does that and lives. At least not very long.
Hmmm. Tough choices. I think I'm gonna have to go with Azazel, though, 'cause Dean warned him, and then he had so many chances to kill Dean but his love of torture got in the way and in the end Dean got him. Killed his son and daughter, too, actually, which really puts him in Darwin territory by removing him totally from the gene pool. Esp. since it was completely his fault that Dean was even alive to kill them.
So yeah. Azazel wins. Can't wait to see Dean get his hands on Lilith, though, that ought to be fun.