I'll limit my list to war movies set in the 1900s- since I'd otherwise have a tough time paring it down and some brief thoughts on some of these flicks.
1. The Killing Fields 2. Taegukgi 3. Saving Private Ryan 4. Platoon 5. Schindler's List 6. Grave of the Fireflies 7. Hotel Rwanda 8. Life is Beautiful 9. We Were Soldiers 10.Flags of Our Fathers
Some movies I hate to leave out so I'll give a brief mention: A Very Long Engagement, Gallipoli, Harrison's Flowers, Apocalypse Now, Black Hawk Down, Saints and Soldiers. I still haven't seen Letters from Iwo Jima (and admittedly have yet to see a classic like Paths of Glory)
Least Favorite War Movies (in no particular order): Windtalkers Flyboys Stalingrad Cross of Iron The Green Berets Enemy at the Gates
Yet I still find all those movies I just listed watchable (except The Green Berets), if that says anything about how forgiving I am towards the war genre.
This is not a list of quality but of the way I was imressed when I saw them for the first time. There are dozens of movies wich are dealing with war. The all time masterpiece "Grave of the fireflies" at the top. But to me war movies always were stories about soldiers in war. (You can call me ignorant.)
Spielberg's "Privat Ryan" changed the way to show war with his documentary style during the "action". But I don't like the movie. "Band of Brothers" is sooo much better. But a tv-show. And "Apocalypse Now"? I was to young when I saw it. I was fascinated but did not understand it. Have to watch it again.
I finally saw Letters from Iwo Jima, such a powerful, moving film. It may be too early and I need a while to mull over the film, but I think it challenges Taegukgi for the #2 spot on my list.