Sorry guys,but I don't know how you think Val Kilmer's performance in Tombstone is anywhere near underrated,considering that he has all the best lines that everyone repeats when talking about the movie to there friends.
His performance was awesome and always gets the respect that it deserves(except from the Oscars,of course).
My choices are Edward Norton in Primal Fear,Sean Penn in I Am Sam,and everything that Daniel Day Lewis does.He's hands down the best living actor,with Gary Oldman not far behind.