As many have mentioned the "Casa Bonita" episode is amazing. I went on a couple of ski trips as a Kid and we would always stop at Casa Bonita on the way.
I think the best thing about south park is how they have transitioned from potty humor into making fun of current trends/events.
Fun Side Story- In 6th grade art class I made a clay cartman figure eating cheesy poofs. The art teacher refused to display it in the schools front display case with all the other kids figures because it was "offensive". I believe it is still at my parents house somewhere, will try and snap a picture of it sometime soon.