Being a child of the 70s I have to give it Vader too.
Heath Ledger's portrayal of the Joker deservedly gets much praise, but I really like Jack Nicholson's Joker too, and of course he was magnificent in The Shining too, although it could be argued that the hotel itself was the real villain.
Other than that:
Christopher Walken and Gary Oldman in True Romance, two AMAZING scenes.
Oldman's channeling of Ross Perot in The Fifth Element too is pretty hilarious.
Kevin Spacey was great in both the Usual Suspects (spoiler alert! ) and Seven.
And Kathy Bates for a female choice was great in Misery.
Daniel Day Lewis in There Will Be Blood is worth mentioning too.
And as I mentioned psychotic charm in my first post......how the hell could we forget the incredible Robert Walker's career defining portrayal of the truly charming killer Bruno in Hitchcock's Strangers on a Train!