Caddyshack: I don't care if I show my kids this when they're 15 or 5, this movie MUST be seen by my kin.
Lion King: Because no childhood is complete without this movie.
League of Extraordinary Gentlemen: Vampires, Nautilus and Sean. Fucking. Connery. They WILL see this.
Ocean's Eleven: To show that no movie has to just be explosions to be completely awesome.
The Avengers ...To show the previous that explosions and flying aircraft carriers are STILL viable for being completely badass.
The last might be a bit quick to judge, but I'm pretty sure it'll still be one of the best movies I've ever seen.