Robert De Niro in Raging Bull. My all time favorite performance.
Runner up would have to be Jack Nicholson in One Flew Over the Cuckoo's Nest and Marlon Brando in a Streetcar Named Desire. I actually don't like a Streetcar Named Desire, but Brando made the movie watchable for me and its probably the single most iconic performance of all time. It changed the way actors...well, acted afterwards.