Tombstone, or The Assassination of Jesse James by the Coward Robert Ford, or 3:10 to Yuma.
Assassination was raging film-making boner. One of the best movies of all time. The only reason why I won't settle with that as my unright answer is because it wasn't a traditional western. It was almost closer to Broadway than Clint Eastwood.
Tombstone had class and realism, but no production value. And while I don't want to blame actors and film-makers for not being able to buy a better movie, it does factor in to a point. Although, I believe it to be Val Kilmer's greatest performance, and that he played one of the best cowboys in the history of film.
3:10 to Yuma seemed to find the middle ground between Assassination and Tombstone, where it wasn't so drenched in money that you couldn't hear the movie past the loud sound of the cha-ching sounds but at the same time it was enough of a money grab for the studio not to let the writers get too original. The actors were pretty good, and the story was great.
But at the end of the day, I haven't seen every Western movie ever made, so I have no ground to stand on, and neither do many of us (considering the possibility that someone reading this has, in fact, watched every single Western movie, whether big budget or indie amateur, ever made.)