Kenny being rejected from Heaven and sent to Hell. The event itself isn't awesome at all, but the gorgeous animation (complete with CGI backdrops of space and Hell and CGI demons tormenting the 2D Kenny) and great James Hetfield song make the whole sequence amazing to watch.
Cartman attacking Saddam and his army with his electric powers, gained due to his V-chip starting to malfunction. "RESPECT MY FUCKING AUTHORITAH!"
Cartman saving Terrance and Phillip by turning off the electric chair's switch and then causing a power surge with his swearing.
The fact La Résistance had the balls to go into the battlefield, full of people dying around them, where they could easily get caught in the crossfire (Stan even nearly got hit by a rocket), just to save Terrence and Phillip. That takes courage for an eight year old.
And Kyle knew full well that he was slowly pressing her Berserk Button hard enough to break the console. He knows he might get killed or otherwise have something horrible happen to him for talking back to her, and that takes guts.
To specify: Chef and the other black soldiers are initially made by the (incredibly racist) general to serve as a "human shield" for the white soldiers (and their vehicles) and get killed first. Their response? Chef gets all the black soldiers to get out of the way so the general's "favored" white soldiers have to deal with the punishment instead!
This brief moment
Sheila: Any last words, boys?
Phillip: Yeah, how about you get me the fuck out of this chair!
This movie (of all movies) getting nominated for an Oscar!
Stephen Sondheim sent a letter to Matt and Trey congratulating them on making the best musical that he had seen in the last 15 years. Trey in particular (who'd been studying music since high school) said that meant more to him than a thousand Academy Awards.
Plus the movie getting into the Guinness Book of World Records for containing the most swear words in an animated film.
Matt and Trey were also able to call out the hypocrisy of the rule in the first place. They basically had their producer, Scott Rudin, kick up a fuss until the final cut was rated R. They proved movie ratings are all about money and power, not the content of the film itself.