Western Writers of America put together a well done (but not perfect) Top 100 Western films of all-time list. Really, the only thing that makes me shake my head in disbelief is the #99 rank for Robert Altman's McCabe & Mrs. Miller, which is one of the greatest films ever made. Here would be my top 10 Westerns, no particular order.
McCabe & Mrs. Miller
Pat Garrett and Billy the Kid
The Assassination of Jesse James by the Coward Robert Ford
The Man Who Shot Liberty Valance
The Naked Spur
The Outlaw Josey Wales
Seven Men from Now