My brief list... All are great films, many are classics. An * indicates an absolute "must see".
Lonesome Dove*
Shane*
Silverado
Unforgiven*
McLintock
Outlaw Josey Wells
The Big Country*
Hondo
The Ox-Bow Incident*
The Man Who Shot Liberty Valance*
True Grit (both original and remake)*
The Streets of Laredo
Broken Trail
Open Range
Crossfire Trail
Last Stand at Saber River
Santa Fe Trail
The Searchers*
The Shootist
Fort Apache*
She Wore a Yellow Ribbon
Comanche Moon
Red River*
Hombre*
3:10 to Yuma
Appaloosa
Quigley Down Under
Wyatt Earp
Pale Rider