The episode where they all get ninja weapons and turn all japanimation and put a star in butter's eye and pass him off as an animal to the vet is fucking great as well.
Actually it's season 8 episode 1, the song where it happens is "Lets fighting love". I can't find the video only clips of the song on youtube...
They need to have a reality show that just does the "making of" southpark, imagine what their round table brainstorming sessions are like, probably fucking epic.