I’ll second Pulp Fiction, Requiem for a Dream, To kill a Mockingbird, and Fargo.
I’ll recommend The Shawshank Redemption, 12 Angry Men, Memento, Eternal Sunshine of the Spotless Mind (best title ever for a movie), American Beauty, Fight club, A Clockwork Orange, Unforgiven, Million Dollar Baby and Dog Day Afternoon.
If you want to catch up to all the TV culture you’ve missed out on I’d watch Ferris Bueller’s Day Off, Dazed and Confused, Star wars, The Good The Bad and The Ugly, Fight Club, both Terminator movies (you can skip the third if you want), The Graduate, Back to the Future, Groundhog Day, Midnight Cowboy (it will help you understand the Seinfeld episode where George buys John Voight’s car) and the number one office movie that dominate the office comedy genre: Office Space.
I won’t recommend Kentucky Fried Movie, although my father made me watch it when I was a kid–so then of course, he figured I had to watch Enter the Dragon…
OK, here’s the list: First Knight
Superman II–skip the rest of the series, seriously A Knight’s Tale
Four Weddings & a Funeral (these last two I just have to watch again periodically. ymmv) The Princess Bride
Groundhog Day
Joe Vs. the Volcano (with your previous viewing experience, you may not be bothered by the low production values) Morgan Stewart’s Coming Home
Pretty in Pink
Metropolis (the silent film by Fritz Lang) Twilight of the Cockroaches (Japanese, mixed animation & live-action, & really out there)