God damnit! I had a whole dissertation typed out on this subject, then lost it. I assure you it was brilliant. I addressed the fact that he did not act in all the movies he has directed (thank you RS, but you forgot Breezy. He pulled a Hitchcock in that film). I talked about the classic Leone films, Kelly's Heros, Joe Kidd, Two Mules for Sister Sarah, the Dirty Harry series. I mentioned Pale Rider and how that film breathed new life into the western genre. I talked about his recent work and how good that stuff is, then I finally decided on which movie I think is his best...
For the story, the supporting cast, and complexity of the character, I like Unforgiven for his best film.