For some reason the Preschool one always kills me. I think it's how the 6th graders say "bewbs". And the one where Cartman does the hand puppet of J. Lo and sings "Taco Flavored Kisses" too. All the songs in that one just give me the giggles because they're so horrible and ridiculous. Oh man, and the one where Butters fakes his death and pretends to be a girl is so good, and the Go God Go episodes with the sea otters in the future... "No, we're fighting a war because the stupid French-Chinese think they have a right to Hawaii!"
See, there are just too many. I probably have 2-3 favorites per season.