Oh, easy one...
South Park: Bigger Longer & Uncut
We went and saw this in the theater twice and I don't think a movie has ever made me laugh so hard. Of course it's crass as hell, but it's also wickedly clever and the songs are genius. We love musicals and all those references just made us love it even more. As the above scene unfolded and I realized it was a riff on "Oklahoma," for example...oh my god. So, so funny. The whole "La Resistance" Les Mis thing is also sheer brilliance.
It's a crime that the Phil Collins song for the Disney Tarzan movie beat "Blame Canada" for Best Original Song that year, but I liked that Parker and Stone took it in stride and used it as an opportunity to make fun of him on their show (toting around the Oscar and so forth, hehe). I just read their comment about it, "We were fully expecting to lose, just not to Phil Collins." Haha... :D