Well, I'm the self-appointed DJ for all of the parties we have at my boyfriend's place, so here are some songs that are must-haves (for better or worse...)
The Buffett:
Margaritaville
Changes in Lattitudes...
Why Don't we Get Drunk...
The Britney:
Baby One More Time
Oops I did it Again
Crazy (the Stop Remix)
(although, some of the guys need to be pretty toasted to tolerate this)
Genie in a Bottle (its a long story, but there's a "performance" that goes along with it)