That's a tough one
A Bridge Too Far is good
So is The Longest Day, Patton, Midway, and of course Tora, Tora, Tora
The Great Escape is damn good as well. Don't really count it as a war movie though.
Still my favorite is probably We Were Soldiers That movie blew me away the first time I saw it and still does. Knew several guys, served with a couple, who were in Vietnam and they were amazed how accurate that movie was. All the way down to the way the death notices were delivered.
Of course Band of Brothers is awesome, but I don't count it since it is so long and wasn't a theatrical release.