I can honestly say that Sergio Leone's (of the 'Dollars' trilogy fame) 'Once apon a time in the West' is the best Western of all time, & I'm sure every single person who has seen that movie agrees. Plus I'm sure that everyone who has nominated other movies will change their mind once they see this movie.
It is the supreme western with Henry Fonda playing the nastiest bad guy ever seen in a western, Claudia Cardinale as the heroine, plus Charles Bronson (I think this is the only movie he actually 'acted' in) & Jason Robards as the good guys. Plus the perfect music score of Ennino Morricone (also of the 'Dollars' trilogy fame). When the film was realeased, critics nicknamed the film 'dancing with death' as the film score was done first & all the acting was done to the music score (like a rock video) so it appeasred that everyone was acting in rythym to the music, including the hoof steps of the horses.
BTW, if you do see it, do it justice & see it in a rerun theatre with stereo sound in wide screen, the family massacre scene near the begining, with its hauting music score must be seen in wide screen, plaease.