I'd rank Johnny Guitar (1954) above all of the Westerns we've covered so far, although I'm not sure fans of the genre would even consider it a Western. Yes, it features cowboys and horses and gorgeous vistas and stagecoach robberies and bar room brawls and crooked lawmen and tense showdowns with menacing villains, but the protagonist is played by Joan Crawford and its dialog is as snappy as the best film noir and its theme is a torch song performed by Peggy Lee.
All I know is, if Karina Longworth (host of You Must Remember This) and Millie De Chirico (TCM and FilmStruck programmer) and Fresh Air's Terry Gross have all independently recommended a film, it's a guaranteed classic.
Here's a trailer that arguably gives away too much of the plot: