What is absolutely the best movie to watch at a party?

Whether you're hosting a movie night, or just want something on in the background while people mingle, there are certain science fiction and fantasy movies that just get things going. What's your go-to movie to get your guests in the mood to party?

Post videos, key visuals, links, and passionate defenses of your party movie choices so that we can figure out if we need one of them during this weekend's outings.