Out side of the Great Escape (WWII) which is in a league of its own I like good Morning Vietnam, full Metal Jacket and Apocalypse Now (all Vietnam war). Black Hawk Down (Somalia conflict?) is ok too.
I know Good Morning vietnam is a bit more of a comedy but it is still epic.
Even though it's technically not quite a "war movie", I'd have to say Tropic Thunder is my favorite. That one and Stripes, even though that isn't really a war movie either...