A lot of good choices on there. I love westerns so I've seen most of them. Sadly, the best of them is The Big Country which isn't included. All star cast, majestic scenery, great score, a wonderful tale about character, and Oscar winner Burl Fucking Ives. A good western should have a moral component along with the action, and there's something about an enlightened man being true to himself in a country full of meatheads that really works for me.
TGtBatU is fantastic, but not something I want to watch very often. It's one you really have to take in, much like The Unforgiven, which I also don't watch very often. Whereas Fistful shows up about once a year. Great fun. It's not as good a film as GBU, but I enjoy it more.
The Magnificent Seven is another once a year flick. Helluva cast and I'm a sucker for anything scored by Elmer Bernstein.
The Searchers was excellent, but you could really sense that there was a lot they weren't telling you. Which was certainly true given the adult nature of the details and the Motion Picture Production Code. It just felt incomplete. Unfortunately, somebody will probably remake it now that they can deal with the adult subjects, but it will have none of the character of the original.
Speaking of the Motion Picture Production Code, The Wild Bunch and Pat Garrett and Billy the Kid are among my favorites. I love that somebody was willing to come along and show you the real awfulness of what was glossed over in earlier movies. The Dollars trilogy was gritty as hell, but it was still highly sanitized. Peckinpah really wanted to show the whole picture, and not just as seen by the good guy.
Didn't care much for Gunfight at the OK Corral, but Sturgis's follow up Hour of the Gun was very good. Whereas gunfight ended with the eponymous shootout, Hour started with it and dealt with the aftermath. Much more interesting story.