Humphrey Bogart in The Treasure of the Sierra Madre
He's paranoid the whole film and Bogart doesn't drop the ball for a second.
Buster Keaton in The General
The picture sums up the greatness
George C. Scott in Dr. Strangelove or: How I Learned to Stop Worrying and Love the Bomb
Peter Sellers is the star of this film but George holds his own tremendously. More great too due to Kubrick tricking him into doing a more eccentric performances. His facial expressions are grand.
Klaus Kinski in Fitzcarraldo
Its another Kinski/Herzog and I rate this performance second after Aguirre.
Victor Sjöström in Wild Strawberries
Captures his character perfectly.
Liam Neeson in Schindler's List
Bit favorable since he is my favorite actor but this is Neeson's best and one of the best I've seen.
I think its a bit early but the past 3 years have had 3 great supporting villains and I imagine they will eventually make these lists. Waltz>Ledger>Bardem is my preference.