Answer:
Depends on who you ask.
MY choices would be Stagecoach, The Searchers, The Man Who Shot Liberty Valance, Shane, High Noon. Butch Cassidy and the Sundance Kid, and Black Robe.
The American Film Institute took a poll of their membership in 2006 and had different results.
More good choices:
"The Big Country" (1958)
"The Magnificent Seven" (1960)
"How the West Was Won" (1962)
"A Fistful of Dollars" (1964)
"For a Few Dollars More" (1965)
"The Good, the Bad and the Ugly" (1966)
"True Grit" (1969)
"Little Big Man" (1970)
"Jeremiah Johnson" (1972)