Crap this is a hard one. Have to go Hoffman though... Papillon, Marathon Man, Kramer vs. Kramer. I just never really emerged myself in The Godfather to understand the hype completely.
@5plitreel - I would have to go with Hoffman as well. He just had the better collection of films in that year era (e.g. Lenny, All the Presidents Men, Straw Dogs, plus the ones you mentioned).