too many to choose just one.
full metal jacket, good morning vietnam, private ryan, 13 hours, glory just to name a few great war movies, and band of brothers if you can count that (I do).
monty python, robin hood men in tights, blazing saddles, tommy boy, animal house, revenge of the nerds, porkys just to name a few for comedy.
godfather, good fellas, casino, scarface, donnie brasco, black mass for some great gangster movies.
snatch, american history x, sweeney todd (yes ive listed multiple johnny depp movies and thats b/c his movies are excellent), one flew over the cuckoos, fight club, some of the Rocky series (emphasis on some) for some good dramas. Another phenomenal dark drama/action movie that received little fanfare was Ronin. If you haven't seen it you are missing out; one of Robert DeNiro's best movies IMO.
indiana jones trilogy, obviously the original star wars 3, the early jaws episodes, predator, rambo, various James Bond series episodes when an actual alpha male played the character (not that pierce brosnan little b*tch), back to the future trilogy, lord of the rings series was pretty good as well if you're looking for good action/adventure type cinema.
For unique, indy type films you have to include flicks like lock stock and 2 smoking barrels, trainspotting, shawn of the dead, reservoir dogs, usual suspects, swingers, a slew of old hitchcock movies like rear window, the birds, psycho, vertigo, notorious, etc. Plenty of cult classics out there, old and new.
Sorry, like i said, no way to pick just one movie, not even one per genre.