I thought it was funny when they were making a list of all the bad things Cartman has done over the years. That, and making amends with Sally Struthers.
Butters blowing it at the talent show killed me! Other hilarious moments: Jimmy's parents announcing that they hear he has been getting erections (every adolescent boy's worst nightmare!), Jimmy's dream sequence and the kindergartener screaming, Jimmy's openings to his jokes:
they elected a new pope, you know about this? do you know what I'm talking about? Did you hear this?
The rip on Italian restaurants that are really chain restaurant crap (e.g. Buca Di Beppo and Olive Garden), nutgobbler, and probably what killed me most...