there are so many to choose from, because the War movie genre has so many sub-genres.
so what about All Quiet on the Western Front
The Longest Day
Lawrence of Arabia
Zulu - woohoo
The spectacular El Cid
Kelly's Heroes for fun
the Charge of the Light Brigade
The Killing Fields
I just can't choose one
It's probably best to break this down to best war movies before 1930, 1931-45, 1946 - 60, 1961-80, 1981-1995, 1996-2011.