I have a few. JR Ewing in Dallas, Alexis Colby in Dynasty, T1000 in Terminator 2 (a true menace), Kathy Bates' character in Misery (a true mentalist). I'm out, anyone else?
Ursula the Sea Witch, that bird from the hand that rocks the cradle, Percy from Green Mile, Denzel Washington in Training Day. Damnit. No, best ever ever ever Bill, from Kill Bill.
Adam Carrington in Dynasty, anyone who can redecorate someone's office with toxic paint deserves the Bad Guy mantle. Oh yeah, and Cindy from Eastenders, for doing such a poor job of try to kill Ian.
Ooh, I think the Child Catcher wins so far, simply because I was absolutely terrified of him as a child...don't know quite how Stephen Gateley's managing at the moment, though...
Nick Cotton? Dirty Den? Andy? According to a recent Eastenders poll, Trevor is the biggest baddie in EE