Brad Pitt is quite the dedicated actor. For 12 Monkeys (most excellent film), he actually studied with psychiatrists and watched videos of schizophrenics [sp?] to get the hand gestures and behavior right.
I am also a fan of Jackie Chan, though he isn't a super actor. He is a great movie-maker.
And I feel that Tom Cruise deserves honorable mention. After watching the behind the scenes on MI:2, I was amazed at what he did in that movie.
As far as raw acting talent? hmmm... that is tough... I would have to say Jim Carey. His raw acting ability is phenominal. I have never been as amazed at a person's ability to disappear in a character as I was when I first saw Man on the Moon. Only after it was over did I remember that it was not Kaufman, but an actor. VERY convincing performance. His flexibility is astounding as well. Many good actors (spacey, deniro) can only portray a single type of character well. Carey can do about anything.