The 'Nam bunch, give me your favorite and why.
Personally, Apocalypse Now. The atmosphere is top notch, and the descent into madness as the movie progresses is amazing. One of the best movies ever. Platoon is also amazing, but sometimes hard to keep up with since there are so many characters dying, getting wounded and so much confusion in the fighting scenes. The Deer Hunter was also pretty eerie, though didn't really work so much with me. Full Metal Jacket has some of the best dialogue ever, but the structure of the movie is a bit oddball. A lot of times it feels like an exposition of "war is hell" scenes stitched together, while a general storytelling thread is somewhat vague.