My all time favorite is ZULU - with Michael Caine.
OK so I named only ONE all time best war movie. Now I cannot control myself in naming some others because while there are none better than Zulu, some come very, very close such as:
Stalag 17 - A POW movie (prisoners of the Germans in WWII) - what a movie - with William Holden! This is quite possibly my second all time favorite war movie.
The Great Escape - cast of many stars
Sahara - a classic Humphrey Bogart
Gunga Din - a classic about another era with Cary Grant et al...
The Dawn Patrol - David Niven - a movie with lots of insight into the minds of those in the air war of WWI.
The Battle of the Bulge - another classic with a cast of thousands
Casablanca - Bogart again - definitely a war movie if not in the classic sense
King Rat - another POW classic this time prisoners of the Japanese with George Segal
Enemy At The Gates - great flick with Joseph Fiennes
Platoon - pretty darned intense
The Four Feathers - the original sound version from the 1930's I think, not the horrible recent remake
The Caine Mutiny - Bogart again (I think it was a wartime flick)
They Were Expendable - The Duke
Paths of Glory - Kirk Douglas
Run Silent, Run Deep - Burt Lancaster and Clark Gable
Patton - George C. Scott
Dr. Strangelove - greatest cold-war film ever with Peter Sellers and with a great George C. Scott role (compare it to his role in Patton).
MASH - the movie
The Mouse That Roared - starring Peter Sellers in 4 or 5 roles; hillarious especially in light of the current events at the time it was filmed. A very short war innthis movies but the outcome is great.
And finally one of which the name eluded for a bit, but I found it through a web search:
Life Is Beautiful - strange name for a war movie. Starring Roberto Benigni. Its about an Italian Jew his wife and his son who were POWs in a concentration camp. The father tried to make it all seem a game to his kid to protect him from the realities of one of the worst results of any war - the horrors of the concentration camps. Yes that too was a war movie, a quite touching and heartbreaking one. One that, if it does nothing else, shows us one of the reasons that going to war is sometimes the right answer.
There are more great ones, but if you get the chance, see all of these for some real variety in types of war movies. These are all very good ones.
All the best,
GB