Has to be Bruce Dern in The Cowboys or Yul Brynner in Westworld both roles gave both actors a major boost in their careers.
I'm not so sure that gave Dern a major boost. I've read several times that he received all kinds of death threats after "killing" the Duke. 
I think three of the most memorable villains were Laurence Olivier as Crassus in "Spartacus", Stephen Boyd as Messala in "Ben Hur" and probably the most menacing of all was Nathaniel Narcisco as Salvaje, the Apache warrior who was Gregory Peck's adversary in "The Stalking Moon". For Western fans, I highly recommend the last title, which will be released on DVD 26 August.
I also have to agree with Rob that Bob Mitchum in "Night of the Hunter" would rank near the top of my list of memorable villains, as would the young man who was Crowe's second-in-command in the remake of "3:10 to Yuma". Or how about Anthony Perkins in "Psycho"? Bela Lugosi as Dracula was always good for a few shivers. Vincent Price too.
