Wow...don't get me started.
Oh, well, ok.
John Fords Cavalry Trilogy: Fort Apache, She Wore a Yellow Ribbon, Rio Grande.
Who Shot Liberty Valance
True Grit, even with Glen Campbell, but both are excellent. Jeff Bridges was amazing.
With a hat tip to Unforgiven, I think Open Range is the best Western we've had at all recently.
For a 21st century Western, I'll plonk down "To Hell or High Water". I like Taylor Sheridan, though being consumed by Yelowstone (which is, just, "ok") is limiting him.
I have never made it through "Once Upon a Time in the West". As renown as they were, I was never a huge fan of the Eastwood spaghetti westerns.
And, of course, the movie so good, they made it twice: Rio Bravo and El Dorado (I prefer El Dorado).