Trouble is it has been so long since I have seen most westerns, so I will not claim this as my definitive list.
1. Once upon a Time in the West
2. Unforgiven
3. El Topo
4. The Proposition
5. Dead Man
Honourable mentions: The Man who Shot Liberty Valence; The Good, The Bad and The Ugly, Cheyenne Autumn, The Searchers, The Outlaw Josey Wales, Hang 'em High, Pale Rider, Appaloosa.
I will not include any of the samurai films as they are not westerns.