The Rocky Horror Picture Show - there really is nothing else quite like it.
Our musical movie marathon included it and I was amazed to find it was first time viewing for a couple of people. It was hugely enjoyed after all Tim Curry parading around in his heels, fishnets and corset is hard to beat as a symbol of complete confidence and the music is just so great. Even now hours later I'm still singing it to myself.
But probably what made it such a stand out for the afternoon was that the preceding movie was West Side Story - now there's a contrast!