First off...I haven't gotten around to The Wild Bunch, Once Upon a Time in the West, Tombstone, or The Magnificent Seven yet...but I will.
Anyway, and this is directly quoted from another thread:
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*High Noon - great psychological and suspenseful western, about a town marshal (Gary Cooper, in one of his best performances) who must get help from the townspeople against 4 outlaws coming to town, but all of the people have their own reasons for not helping; a fascinating examination of morals, and one of my favorite films
*The Good, the Bad, and the Ugly - excellent western taking place during the civil war with Clint Eastwood, Lee van Cleef, and Eli Wallach as 3 outlaws forming an uneasy allaince to recover gold; the great score and fantastic camerawork of the Old West (actually filmed in Spain) make this film - watch for the 5 minutes leading up to the gunfight at the end
*The Man Who Shot Liberty Valence - I can't say enough about John Ford, but his last great Western really takes everything he did before and builds upon it in this fascinating study of the skewing of the line between fact and legend, and civilization and wilderness; I recommend seeing some other John Ford Western's before this though, including Young Mr. Lincoln, My Darling Clementine, and Fort Apache, before seeing this one - you'll be able to appreciate it better that way; stars John Wayne, Jimmy Stewart, and Lee Marvin as the 3 title characters (if you've seen the film, you know what I mean)
*Butch Cassidy and the Sundance Kid - classic buddy film about 2 bank robbers, Paul Newman the intelligent planner who has never killed a man, and Robert Redford, the gunslinger, who need to flee to Bolivia to avoid capture by the Superposse, a group brought together by the president of the Union Pacific railroad; one of the very best passing of the West films (the Superposse is a symbol of the advancing times and technology)
*Red River - John Wayne stars as an early prototype for his later Ethan Edwards character, and Montgomery Cliff is his adopted son; the best of the cattle drive films; in a way, this is a western version of Mutiny on the Bounty, but this time Fletcher Christian is Bligh's son; Wayne vows to kill Cliff, but once he finally catches up with him...well, the ending is a bit weak, but everything else is great
All of these films, as well as The Searchers, are often contenders for the title of the Greatest Western Film Ever. I highly recommend all of them.
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However, currently, The Man Who Shot Liberty Valence gets my nod...I'll mention the scene where Strother Martin, in that weird voice of his, says "I'll get it, Liberty!", regarding a steak that John Wayne wants Liberty to pick up, and Wayne kicks him in the face.