OK, Lukas, I'll opine.
For me there are several conditions that must be met to make a Great War movie.
First and foremost it must be a compelling story within that war's context. But it still needs to be a "War" movie. Simply documenting what happened in the particular war does not cut it. That's recreating a documentary. That knocks out movies like Tora Tora Tora, Midway and many others. It also eliminates my number one movie CasaBlanca, which is not a true "War" movie.
Secondly it has to be about a real war (sorry Lukas, Starship Troopers is eliminated. It also eliminates the great movie, Dr Strangelove! )
It has to have a career defining performance by a great actor or great performances by an ensemble cast (sorry again, Lukas, this knocks out movies like Kelly's Heroes, Donald Sutherland, a WWII hippie, really?) I think the story line has to draw us in and make us really care about the struggle and the outcome even though we already know what it is. For most, this eliminates movies like Platoon. Do you really care about the characters in that movie? No, and that was Stones point, but it doesn't make a rear "War" movie. Maybe a great political statement, but not a Great War movie.
The dialogue should be authentic, Unfortunantly that eliminates a lot of older movies for me and some newer ones as well. The dialogue in "Enemy at the Gates" was atrocious! Besides listening to British Dialects spoken by Russian soldiers just made my skin crawl.
I think the cinematography needs to be memorable. That is to say I think the filming needs to truely transport you to place, the battlefield, the outpost, whatever. This eliminates such movies as John Wayne's The Alamo.
Often overlooked and imperative, I believe, is great scoring and an iconic Movie Theme. There is a lot of great scoring but when a movie rises above the fray, it elevates the music. A clear indicator of a great movie.
So which movies meet all my prerequisites?..... Patton, Saving Private Ryan, Apocolypse Now, the Good the Bad and the Ugly, Bridge on the River Kwai, All Quiet on the Western Front, Gallipolie (my sleeper war movie) and many others.
But the one that stands out to me as just a head above the others is...... "LAWRENCE OF ARABIA"!!!!!