that is, here's my ongoing list of twenty:

Destry Rides Again
Stagecoach
My Darling Clementine
Red River
The Cavalry Trilogy:
    Fort Apache
    She Wore a Yellow Ribbon
    Rio Grande
Winchester '73
The Searchers
The Magnificent Seven
The Man Who Shot Liberty Valence
Ride the High Country
Cat Ballou
Once Upon a Time in the West
The Wild Bunch
Valdez is Coming
Little Big Man
The Outlaw Josie Wales
Silverado

most have already been mentioned, new additions being Rio Grande (the third and least known of John Ford's trilogy), Valdez is Coming (with a screenplay by Elmore Leonard), and Silverado. interestingly, seven of them are by John Ford, and seven feature John Wayne (who isn't a particular favorite of mine).