For me, the boundaries of the genre are pretty narrow. Despite how much I may love a given flic, I won't tag a drama as a war movie just because it unfolds in the midst of a war. Accordingly, I won't include favorites like 'Empire of the Sun' and 'The Deer Hunter'.
Elements/themes that I look for in War movies are: overcoming fear/cowardice, loyalty, unit cohesiveness (including all sorts of prejudice), and -- of course -- accomplishing the mission. A great war movie also tells something not just about the characters, but about the conflict. I don't like shallow glamorizations.
My three favorites are (all Army):
Platoon
Sahara
Black Hawk Down
The two David Lean epics -- 'Bridge over the River Kwai' and 'Lawrence of Arabia' -- meet the criteria above but merit special consideration. IMO 'Lawrence' is bigger than a war movie because T.E. Lawrence's accomplishments were larger than life. Kwai is a great movie but problematic in that I feel it tries to do too much (in posing moral dilemmas) and suffers dramatically as a result.