I just watched Jean-Pierre Melville's Army of Shadows and thought it was really, really good. It's a WWII French Resistance movie, but it doesn't really use any WWII/Resistance movie cliches (I think Melville was actually in the Resistance for the reals). If you want something that's a little more fun, I like Thunderbolt and Lightfoot. It's a Clint Eastwood and Jeff Bridges proto-buddy-action-heist movie. Pretty weird.
Oh, and I slogged my way through Once Upon a Time in America, and Pat K is right. It's not that good. Actually, I really like all the stuff about when the characters are kids, but that's only like 30 minutes of a 9 hour movie. Oh well.