Sergio Leone single-handedly directed seven movies, and five of them were westerns.
Sure, John Ford is probably the more prominent director when it comes to the western genre (there's even a whole book written about the subject called John Ford Made Westerns). But, it's hard to deny that Leone shouldn't also be put in GOAT consideration when it comes to directors of the western, especially with movies like The Good, The Bad, and The Ugly on his resume.
