I understand the votes for Jack Nicholson, but I'd like to throw in a different candidate - A Few Good Men.
A film packed with incredible writing (Aaron Sorkin is a genius) and acting from a superb cast (Tom Cruise, Kevin Bacon, Demi Moore...) but absolutely outclassed by the incredible Jack...
Robin Williams in Good Will Hunting is incredible, the depth and power of his performance is incredible for a comedian at heart. His dramatic performances were always incredible. Such a sad loss.