I can't pick one, but here's a short sample of my preferences:
12 O'clock High
300
Ballad of a Soldier
Band of Brothers
Catch 22
Dark Blue World
Der Untergang (Downfall)
For Whom the Bell Tolls
Gettysburg
Lord of the Rings
The North Star
Ran
Saving Ryan
Tae Guk Gi: The Brotherhood of War
The War Lover
War & Peace (the Russian-French 5 hour flick)
Waterloo