I'm bad at this. I love villains way too much. Bastards are the best.
I think my character from Fallout 3 may be up there as one of the evillest though. I mean, upon discovering that she couldn't murder children, she then proceeded to kill kids' parents and eat their still warm flesh while the children watched. And they were tasty. Most villains don't get to be so creative.
As far as scripted characters, though...Hmm...My standards are evidently weird, but the only ones I can think of putting here are the Executive Glukkons from Oddworld. They use slave labour and murder sentient creatures for tasty snacks, and then stitch their eyes and mouths shut so they can't see their secrets. The bastardry is totally institutionalised. Yeah, the Glukkons totally take the cake.