Geoffrey Rush in just about everything I've seen him in. Like Sir Ben Kingsley he's one of those actors whose own persona doesn't overwhelm the character.
Also impressed with Michael Sheen in the Queen and The Damned United. I haven't seen Frost /Nikon but I believe he was equally good in that, one of those actors who becomes the person they are portraying.
I agree that Heath Ledger would have gone on to be an one of the best of his generation but I preferred him in films like The Knights Tale and Candy (in which you get another great performance from Geoffery Rush as well)to the Batman films.