We're shaped by our thoughts and sentiments. Our thoughts and sentiments are shaped in large part by experience and knowledge. Movies affect both of these whether we watch them on large screens or small. What most of us think and feel about the American West comes from the images and words seen and heard in cinematic portrayals. One of the most enjoyable aspects of doing my homework for West from Yesterday was the discovery of how much about the American West I didn't know. This list is not comprehensive, chronological or in descending order. These are simply films I like a lot. Some I love. A disproportionate number have an "end of the era" theme (The Wild Bunch, The Professionals, Open Range, The Shootist, Monte Walsh) but one thing they all have in common is dirty shirts. Even as a kid, I was suspicious of portrayals of characters in general, cowboys in particular on whom dust, dirt and sweat were conspicuous by their absence.
If you think I missed an obvious candidate or simply don't understand why I included a particular work, let me know. And if you think this list is too big, take a look at Wikipedia's list of western films to see just how many there are.
Lonesome Dove (Okay, it's a television production but still a best-in-class)
The Wild Bunch
The Ox Bow Incident
Lonely Are the Brave
No Country for Old Men
The Big Country
Monte Walsh (the Lee Marvin, Jack Palance version)
The Unforgiven (the Burt Lancaster, Audrey Hepburn version)
Last of the Mohicans (Technically not a "western" but certainly a 'grandfather" of westerns in theme and scope)