i can agree with many choices here, i'll mention the ones that i'm really fond of
* Unforgiven (amazing movie, more of a character study than a traditional western but with some amazing scenes, the final confrontation, the atmosphere, Clint's speech about what he is, magic)
* The Good The Bad & The Ugly (in a lot of movies, the scene were Tuco gets to the cemetary and finally finds the grave would just have been a piece of exposition, probably him riding on a horse, seeing the graveyard from afar, then a dissolve to him walking up the grave and that would be it => here Leone transforms it into art, a perfect fusion of image and music that has a lot of deeper meaning, it helps that EVERYTIME i see this scene, or even only hear the music, i get goosebumps)(while Once Upon A Time In The West is amazing as well, for me it falls just under TGTBTU, which is Leone's masterpiece)
* Django (1st time i saw this, i didn't really like it and found that the chaingun scene after 30 minutes in was the high point, but seeing it again on Bluray a couple of months ago, this is a movie where everything just fell into place, amazing music, acting, look and violence)