Other than the aforementioned Monty Python, here's a few others and going off anything 15 years or older:
Tombstone
True Grit (the original)
Unforgiven
Terminator and T2 (forget it after that)
Raiders of the Lost Ark and The Last Crusade
The Godfather series
Goodfellows
The Magnificent Seven
Close Encounters of the Third Kind
Raging Bull
Rocky 1-4
Bridge on the River Kwai
The Longest Day
A Bridge too Far
The Dirty Dozen
Blade Runner
Spartacus (the original)
The Good, the Bad and the Ugly
Citizen Kane
Gladiator (just makes the cut)
The Shining
To Kill a Mockingbord (the 1962 version)
Heat
The Great Escape
Predator
First Blood
Enter the Dragon
Die Hard 1-3
Dr Strangelove
Forrest Gump
Saving Private Ryan
The Usual Suspects
A good list to get this current generation started.