When looking at the very tops, I have a tough time choosing between The Good, The Bad and The Ugly; or Once Upon A Time In The West. Sergio Leone was a freaking master at the western. I might have to chose Once Upon A Time...simply because of the beautiful scenery and the spectacular cast (Robards and Bronson were never better, and Claudia...wow). But Fonda stole the show.
Tombstone was a lot of fun.
The Wild Bunch had three remarkably strong scenes, including the final shootout.
High Noon was a great psychological examination...as well as the spectacular "Man Alone" shot.
And Unforgiven was breathtaking, and a great (seemingly) western swan song for one of the great western actors and directors.