Help me out here, f-list

Question from movie_greats:

"People are always compiling lists of the greatest movies of all time. But what about the greatest performances of all time? What would you pick as the five greatest performances of all time by an actor, and the five greatest performances by an actress?"

I love the question, but I'm not sure I can answer it without a little help. Give me some nominees in the comments, please, fellow movie buffs!