'We Were Soldiers' and, of course, 'Apocalypse Now' both rank high-up there as good ones to watch that everyone recognizes and should just for sheer pop-cultural value (Apocalypse Now especially), but also because I think they do a good job of presenting the different 'faces' of the Vietnam war, even while broadly presenting similar anti-war themes. Kubrick, of course, goes reflective and melancholy/pessimistic through the whole thing. While Wallace does as well, there's a considerably different and more sympathetic tone. I honestly think they make good 'companion pieces' as far as films go.
On related note, anyone who hasn't read 'We Were Soldiers Once, And Young' or its follow-up by Galloway really should.
On a less well-known note, 'The Beast of War' (or 'The Beast'? I'm unsure which is actually the title) would be one I'd throw to people. It's a late-80s movie set during the Soviet-Afghan war with a Soviet tank crew and their opposing mujahadeen as the primary characters. I have no idea how accurate any of the military doctrine or behavior is, but its a character study more than anything, and the divides presented between Afghan tribal-members themselves, the PDPA Afghani who works with the Soviets, and the Soviet crewmen themselves are enough to drive the movie.
Oh yeah! And 'Waterloo' is one that should be checked out just for the sheer scale and scope of it as accomplished with practical effects. So many extras and so many wide, panning shots from a helicopter of infantry squares and Napoleonic line-battles it's just impressive to watch.