Most of these are going to be pretty well known but most of the best films are.

Once Upon a Time in the West
The Third Man
The Night of the Hunter
Judgement at Nuremberg
The Hustler
The Manchurian Candidate
The Spy Who Came in from the Cold
The Sand Pebbles
Z
The Last Picture Show
The Day of the Jackal
The Stunt Man
Southern Comfort
American Pop
Runaway Train
A Fish Called Wanda
Ran
Once Upon a Time in America
A Bullet in the Head
The Killer
Hard Boiled
The Aristocrats
Running Down a Dream (Peter Bogdon...however you spell his last name, Dorothy Stratten's white knight wannabe who directed The Last Picture Show's documentary on Tom Petty & the Heartbreakers)

In the case of all of the above go with the original and stay away from fecestastic remakes starring Denzel Washington, Bruce Willis or Slyvester Stallone.

Now I am assuming you have already seen the following.

The Maltese Falcon
The Treasure of the Sierra Madre
The Bridge on the River Kwai
Lawrence of Arabia
It's a Mad, Mad, Mad, Mad World
2001: A Space Odyssey
The Searchers
The Wild Bunch
Patton
A Clockwork Orange
The Godfather
Chinatown
The Godfather Part II
Jaws
Marathon Man
Pulp Fiction
The Usual Suspects
L.A. Confidential
Monty Python and the Holy Grail
Blazing Saddles
The Last Waltz
GoodFellas

because who hasn't? If not then I hereby sentence you to a double feature of St. Elmo's Fire and Wall Street with audio commentary tracks of FatBloodyFingers giving a dissertation breaking down how these films represent the exploitation of the poor and middle class by the economic policies of Ronald Reagan and the amorality of the parasitic Yuppie scum of the 1980s.