"Once Upon A Time In The West" - Magnificent film, set the gold standard for westerns IMHO.
Misty - Not old enough, but i did see the longest day a few weeks back and was gob smacked at its breadth (both allied and axis POV) and depth (historical accuracy - compared to others {yes, "Battle of the Bulge" i'm talking to you}, storytelling, and characters).
The Caine Mutiny is also amazing (not old enough - but being a history buff, i love good quality war movies). Though Bogart's acting is a bit off-putting at first - but by the end of the movie it makes sense...
For a change of pace "Idiocracy" is brilliant. Mike Judge ("Office Space") is a genius.