As far as "great" movies go, few rival Unforgiven IMO.
Others that I haven enjoyed in the more modern (let's say post-'80) would be some pretty ridiculous one's like Tombstone and The Quick and the Dead. Let's face it guys, we wouldn't watch that one twice if it wasn't for Sharon in her prime. Open Range, 3:10, Hateful 8 was pretty good, and Django was just silly but still entertaining as all Tarantino movies are. I liked True Grit with Bridges better than the original.
As far as classics? Well... Rio Bravo, Liberty Valance, all the Eastwood flicks. let's just say I enjoy the genre as background or sleepy Saturday fare. So many of them seem the same I can't remember which are which in most cases. Things like Burt Lancaster or Vic Mature playing the parts of American Indians is pretty hard to not chuckle at, but it's what they did then.