I think Indiana Jones Raiders of the Lost Arc. I love that film. It would be a very hard to one choose one. I picked Indiana Jones because it has comedy, action, and romance. I am a fan of Temple of Doom too which I guess most people don't like as much. I like temple because it has the nice tunnel scenes, the bridge scene, and short round. Temple was very close to being as good or better then raiders.
If I could choose more films I would pick Fight Club as one of my top picks. For some reason that movie is one of my all time favorites. The Matrix would be a cool movie to send to the aliens. It probably would scare them.