I'm a fan of more art house porn so if we're looking at the classics I gotta go with Green Door.
Honorable mention to Cyrano. I looked it up and it's 1991, so I guess it doesn't make the cut, but this movie is legendary among my friends.
We found it on VHS behind the Little Caesars in on the west side in Olympia, Washington in a brown paper bag with a few cans of cat food. It's hysterical. We "discovered" it over 10 years ago, brought it with us across multiple states and still pass it around to this day.