Khan. I mean come on... the guy could quote Shakespeare and still sound cool. I think we almost have to look at Khan as a given. The real question is who's number TWO.
I'm torn, but I'm inclined to go with Lore simply because his capacity for evil was virtually unbounded. But perhaps the greatest of evils is betrayal. To that end I would have to put Captain R. M. Merrick in for a nomination.