So I thought I'd come up a few other movie party scenes of note:
General - Fellowship of the Ring, Boogie Nights, Kiss Kiss Bang Bang (I can't find the clip I want, but here's another party), Dirty Dancing, Empire Records, Spring Breakers (probably NSFW), Velvet Goldmine. The best go to: Almost Famous and Rocky Horror.
School Parties - Clueless (best clip I could find), Sixteen Candles, Can't Hardly Wait, 10 Things I Hate About You, Rules of Attraction, Kaboom (can't find a clip), Romy & Michelle's High School Reunion. And the winner is:
Animal House. Obviously.
Halloween parties - Trick 'r Treat (NSFW), Legally Blonde, Scream
Dance/Bar/Club scenes - Night at the Roxbury, Black Swan, Babel, and then Queer as Folk gets an honorable mention, because goddamn. Was there a theme party too extravagant for Babylon? There was not. (I'd hope it would be self-evident but that link is probably NSFW.)
Masquerades - Phantom of the Opera, Sweeney Todd, the Man in the Iron Mask, Casanova (no clip), The Dark Knight Rises, Snow White
Balls/aristocratic shindigs - Ah yes. The best part of any Jane Austen type period piece. Napoleon (a good chunk of the third quarter is a party), Marie Antoinette, Shakespeare in Love, A Royal Affair, Young Victoria, Emma, Pride and Prejudice, Becoming Jane, really the entire Jane Austen cannon, Elizabeth, Romeo and Juliet (ignore the dubbed ballet music), Anna Karenina, The Duchess (music only), and getting back into the swing of Gatsby:
Bright Young Things
New Years - 200 Cigarettes, When Harry Met Sally, Sunset Boulevard (wait... make that worst party. Same for the Gold Rush.) An American in Paris wins (non-embeddable)
And we won't get into dinner parties, as that usually goes in a whole other direction.
What am I missing? Party Monster? 24 Hour Party People? Mean Girls? Wedding reception scenes?