I love the genre, and I'd find it very difficult to nail down a favorite. But I'll post a couple I've seen posted that I'd like to add a bit to, and some that I didn't see mentioned that I think some might really like.
- Stagecoach (There are so many John Wayne options, that I'll just go with this one)
- No Name On The Bullet (The way the hero and villain roles are set all in a jumble is something I always find worth watching when this comes on)
- Johnny Guitar
- The Naked Spur
- The Wild Bunch
- Destry Rides Again
- Shane
- Winchester '73
- Ride The High Country
- The Ox-Bow Incident
And, for one that might be off the board, you might want to check out "El Topo".