I was about to say that for me it's a tie between Gary Oldman and Daniel Day-Lewis, though DDL is apparently semi-retired from acting.
A lot of popular actors play more or less the same character in every film. As far as I'm concerned, that's not acting, that's being a film personality. Good actors should be chameleon-like, barely recognizable in personality from one role to another. Another good example of this is Ben Kingsley - watch him in his idealized, saintly portrayal of Gandhi, and then watch him as a brutal London gangster in Sexy Beast and it's hard to believe that it's the same man.
My favorite performance from Gary Oldman was in “Leon” aka “The Professional”. He was a true monster.
Daniel Day-Lewis doesn't act. He just metamorphosizes into the character. ;-)
“A lot of popular actors play more or less the same character in every film. As far as I'm concerned, that's not acting, that's being a film personality.”
They call it “typecasting”. Some movie stars make a great living being themselves. Some actors do such a nuanced performance that most people can hardly tell they've adapted to the character. Jack Nicholson comes to mind. He's won more Academy awards for acting (as reviewed by his peers) than anyone. If you watch him do an emotion like anger, you'll discover that he does it differently in every performance. He really does have a lot of range even while remaining recognizable. But I do appreciate actors who vanish into their performances.
You may appreciate a famous scene on the topic from the movie “My Favorite Year” (a movie about the making of a TV show). The show brings on the swashbuckling actor Alan Swann (played by Peter O’Toole), who is accustomed to the retakes of filmmaking but is blindsided when he discovers his television appearance will be live. Overwhelmed by the pressure of performing live, he panics and declares, “I'm not an actor, I'm a movie star!”.
And, if you're into trivia, you might also appreciate that, in 1982, Ben Kingsley won the Academy award for best actor for his role in "Gandhi", beating out Dustin Hoffman in "Tootsie", Jack Lemmon in "Missing", Paul Newman in "The Verdict", and Peter O'Toole in "My Favorite Year".