Lots of excellent suggestions.
Depends what you're after, he said stating the bleedin' obvious. But...
The Graduate - seriously cool, every frame could be a still, quintessential '60s
Butch Cassidy & The Sundance Kid - I can't imagine anyone not enjoying it. "Who are those guys?"
The Philadelphia Story - a personal favourite. Brilliant script, immaculately performed
To Have and Have Not - Bogart at his best, Bacall a stunning debut, at 18
Singing in the Rain - if you don't get happier, check for a pulse
Lawrence of Arabia - big screen is a must. Big, big screen.