On the comprehensive movie list site, They Shoot Pictures, Don't They?, John Ford currently ranks #4 on the list of the all-time 100 greatest film directors (with Orson Welles, Alfred Hitchcock and Federico Fellini ahead of him), though he has placed more films than anyone else, 18, on the list of the all-time top 1000. I think the reason he doesn't rank higher is that he was one of the few great film directors to be fully appreciated in his own time. He won the Best Director Oscar four times -- still a record -- and took home an additional two Oscars for his wartime documentaries.
Welles was once asked whose films he studied when he made Citizen Kane in 1941, and he replied: "the old masters, by which I mean John Ford, John Ford and John Ford." Of course, even by the time he was an "old master," Ford would continue to make films like They Were Expendable, My Darling Clementine, The Quiet Man, The Searchers and The Man Who Shot Liberty Valance. It's no fun, when making lists, to mention people who are already so well covered.