(I just made a list of ten):

Medium Cool (1969)
dir. Haskell Wexler - Wexler made a name for himself through innovative cinematography, but here he directs a docu-drama which captures the social and political chaos of late sixties America.

Wesele / The Wedding (2004)
dir. Wojciech Smarzowski - a wittily black Polish comedy which piles on upheaval after upheaval in an almost surrealist fashion; it is the kind of film Godard may have once made, in which the protagonist, a charming and despisable millionaire, solves all his problems with money, of which he has an endless supply.

Kongekabale / King's Game (2004)
dir. Nikolaj Arcel - a moody political thriller in the vein of All the President's Men which I saw on the big screen back in 2005 after seeing three average films back-to-back-to-back before it.

Crossfire (1947)
dir. Edward Dmytryk - visually gorgeous, excellently acted film on racial tensions; very daring masterpiece.

Build My Gallows High / Out of the Past (1947)
dir. Jacques Tourneur - one of the fastest, wittiest and cleverly structured noir films I've seen. I suspect older members have seen it - and love it.

The Window (1949)
dir. Ted Tetzlaff - gripping, tense B-movie thriller which must be praised for visual superiority and brevity of storytelling.

The Fall of the Roman Empire (1964)
dir. Anthony Mann - Mann's known more for his Westerns, and for some reason, this epic never gained the cultural or historical momentum of others made in the fifties and sixties. See it, for its outstanding cinematography, production values and a story from which Gladiator derives much.

Westworld (1973)
dir. Michael Crichton - Everyone knows Yul Brynner from The King and I and perhaps The Magnificent Seven, but as a kid who saw neither of those films (and still hasn't), Brynner was always the face of "Westworld", the holiday resort which takes people back to the Wild West, with murderous malfunctions to boot. It's a great film rich in allegory.

The Outfit (1974)
dir. John Flynn - A wonderful, tough-talking machismo thriller with Robert "Tom Hagen" Duvall, who made a career of stealing films from brief scenes and bit parts - here he carries the film alone.

Paradise Alley (1978)
dir. Sylvester Stallone - Two years following Rocky comes Stallone's directorial debut (a year prior to the first Rocky sequel), an odd little tale of Little Italy's Hell's Kitchen, brotherly friendship and wrestling. One of the most memorable, rain-drenched climaxes I can think of.

^^ If any of these take your fancy, definitely check them out.