I agree with many already listed, especially the more light-hearted films like "Support Your Local Sheriff", "Cat Ballou" and "Cheyenne Social Club" (Stewart and Fonda play off each other brilliantly). I recently saw "The Assassination of Jesse James by the Coward Robert Ford" and was quite impressed with Casey Affleck. I also agree with "One-eyed Jacks", "Hombre" and "The Oxbow Incident". And I don't know why David E disparages "Little Big Man". I liked it. Here's some that haven't been mentioned: "Missouri Breaks" with Nicholson and a totally outrageous performance by Brando; an earlier version of the life of Roy Bean called "The Westerner" starring Walter Brennan and Gary cooper; I have to admit to liking Michael Cimino's notorious "Heaven's Gate", the movie that brought down a studio with it's directorial excesses and poor reception by the public. Rated as the 6th worst movie of all time I still like it. Maybe because it's based in the true story of the Johnson County War between powerful cattlemen and homesteaders, the archetype of a story repeated time and again in movies and literature and real life, class struggle in the old west.
   But lastly I have to mention "McCabe and Mrs. Miller" starring Warren Beatty and Julie Christie. It's a fine movie with personal
significance because it introduced me to the music of Leonard Cohen.