Like most on this board, I've also seen all the John Ford movies, many times each. They are the kind of movies that I will just see over and over, not only because of the Duke's nearly overpowering leadership presence on the screen, but Ford's ability to portray the old west the way I think it was and never will be again. The Duke and Ford, together, were a team the likes of which we will probably never see again; at least not in my years.
For a different sort of reason, having nothing to do with why I like all the John Ford westerns, not to leave out "They were expendable," I thought among all of the Duke's movies, one of, if not the best was "The Shootist." You'll remember it as the story of an aging gunfighter, fighting TB and living out his last days. When I first saw it about the time it came out (1976, I think), I had the impression the Duke was playing himself, considering his own health at the time.
Take a look at http://www.glendonswarthout.com/films/shootistfilm.htm for a good description of The Shootist." There, you will read, "The Duke goes out with his boots on and his six-shooter smoking, exactly the way Western movie heroes should. This now classic Western remains a fitting final tribute to an authentic American legend."
I think there's no doubt, that during the filming of The Shootist, the Duke knew all too well that would be his last movie