Has anybody mentioned Destry Rides Again?
That's a fun western, where James Stewart almost manages to prove that you don't need a gun to be an effective lawman in a western.
I'd also like to mention Yojimbo . As I understand it, the film was inspired by westerns, and for those of you who haven't seen it, if you do watch it the plot will seem strangely familiar ......
Other than that, I can only echo what everyone else has said - especially about Unforgiven and High Noon.