Wes Studi as Magua in The Last of the Mohicans was down right terrifying:
Ricardo Montalban as Khan I think was the best ever, and one of the most under rated performances in the history of film. I don't think a guest actor ever stole the show quite like Ricardo did in ST2. The ultimate Trek villain:
I think my big thing with these two, is both of their characters were very easy to sympathize with. They had a legit bone to pick with the protagonists. The more I watch ST2, the more I wish the Enterprise never got away and Kirk and his whiny kid could have just been vaporized.