I love the old "cast of thousands" war movies from the just after WWII through the early 70's.
Battle of the Bulge (old one is better),
Midway (both are good, old one better)
Kelly's Heroes
The Dirty Dozen
Inglorious Bastards (1978/2009 both good)
also - Band of Brothers.
Vietnam War movies:
Platoon
Apocalypse Now
We Were Soldiers
Boys from Company C & Full Metal Jacket - same basic movie decades apart, both good, and R. Lee Ermey is the DI in both movies
Siege of Firebase Gloria
Hamburger Hill
Good Morning Vietnam
Random stuff
Braveheart
The Patriot
Heartbreak Ridge
John Wayne's war movies.