And if you somehow manage to find a weakness, it could just go back in time and warn itself. Or gather an army of Shrikes to kill you.

To contribute, I would have to say Dracula. A lot of people I've talked to don't seem to like this book and I can't see why. The way he isn't an overt monster, like the movies portray him to be, gives him a more chilling personality. On top of that, he's highly intelligent and cunning.