Grease is a stupid fucking movie and even the people who said it's "classic" (Mr Brainlove, Mr Rocknrollmassacre..!) know it's a stupid fucking movie. Next you'll be saying ONJs Let's Get Physical video is a classic. OK, that one is a classic, but c'mon!
For musicals there really is only one. When I was a kid, back in the nickelodian days, we'd go to the theatre and see the Rocky Horror Picture Show. The Balboa theatre had a stage and these crazy fuckers would all dress up like the characters and act out the movie as it played. "It's just a jump to the left!" ...When the movie mentioned 'rice' everyone in the theatre would throw rice. In the rain scene, people would pull out squirt guns and shoot water everywhere. My favorite part was throwing toast. And, of course, every one was loaded beyond good taste. It was great!