John Wayne in The Searchers.
Wayne was so completely cast against his usual role of everybody's tough but warmhearted hero that you can really see that the man could act, and act well. He got nominated for Oscars for more than a few pictures and won for True Grit (another great western), but it was in The Searchers that we truly see the depth of his ability. We didn't just dislike his character - we hated him! Here's our old hero so full of violence and hatred that he's about to kill his own neice simply because she's been held captive by the Apaches and is now "tainted goods". His struggle to overcome that hatred and answer the call of blood and family is a a masterwork of his craft.