Grave of the Fireflies
Will never watch it as I value my peace of mind too much and having been told the story, I know I wouldn't be able to cope.
It's legitimately one of the best films ever made, but it's just devastating in a way that no other film I've seen is. Disney didn't want it when it bought the rest of Studio Ghibli's films for US release, proving that it's some what out of place in this thread, but any film fan who thinks they can stand it should definitely see it. Probably the most powerful anti war film ever.