Many people have made good choices. I'll bet we've missed a few, too. Here's my contribution (in no special order):
- The Guns of Navarone
- Gettysburg
- A Bridge Too Far
- The Great Escape
- The Enemy Below
- Stalag 17
And some movies that were not necessarily "military" movies, but were war-related, anyway:
- The Good, the Bad and the Ugly
- No Time for Sergeants
- The Last of the Mohicans (The Daniel Day Lewis version)
- Spartacus