I was watching some poorly made countdown show on Reelz channel, which is the only network that I've seen that gets its ad revenue entirely from as seen on TV ads, that was counting down the top 10 most patriotic movies, of course for the 4th of July. The list was awful, and the show as basically just a series of clips; nothing was said about the movies, and I'm guessing they put on whatever films they got the rights to. So, I feel a need to balance out the universe.
I joked while watching it that the most patriotic film of all-time, since it didn't specify to which country it is patriotic to, was The Triumph of the Will. As far as England goes it's a toss up between Mrs. Miniver and Henry V (more so the Olivier version). If one goes by the film with the most American flags in it, Rocky probably wins (and Rocky was on their list for some reason and not Rocky IV).
So, I'll here specify American films. But this is still too vague as under this criterion I'd say birth of a Nation has a shot, since it was uber-patriotic to a certain (and scary) version of America. The same can be said of the Indian genocide operas: Stagecoach, The Plainsman, or Fort Apache.
That said, the most patriotic American film made, by the "go America!" definition is probably Frank Capra's Why we Fight series, a propaganda masterpiece that at times bested even the Nazi films. However, today its rather embarrassing.
In thinking of possible dark horses, I'd say a strong case could be made for the Godfather as a patriotic film.
So, I'll just list the 3 movies that make me feel proud of my country:
Honorable Mention: Six String Samurai, 1998, dir Lance Mungia
Wha? Stay with me...Buddy Holly traverses a post-apocalyptic USA after its been nuked by the USSR, to succeed Elvis as the King of Las Vegas, the last that remains of America. It's a road movie, it's a rock 'n roll movie...seriously this is as American as it gets.
3. The Grapes of Wrath, 1940, dir. John Ford
It's one of the great films about America from one of the great novels about America. The picture of America is not always pretty, but it shows how even at our worst, there is a sense of right and hope that can rise to overcome any occasion.
2. Bad Day at Black Rock, 1955, dir. John Sturgess
Spencer Tracy plays a one-armed stranger who drifts into a small town that has a dark secret that he's determined to expose. One of the best films ever, and one of the best films about law and justice ever made.
1. Eyes on the Prize, 1987, dir. Henry Hampton
One of the greatest American epics told on film, this mammoth yet riveting documentary masterpiece reconstructs and tells the story of the Civil Rights struggles as told by those who lived it. After over a decade of rights issues the first half of the series is finally on DVD.
After I posted this I found this list from the AV club which is pretty good as well.