Probably Asspen, where they visit the ski resort. Aside from being funny, it introduced one of the few memes I actually use in real conversations. Spoiler <iframe width="560" height="315" src="//www.youtube.com/embed/6Ls5j5iz2eA" frameborder="0" allowfullscreen></iframe> Honorable mention to Cartoon Wars, and the Black Friday trilogy
I think I might like the LOTR or the 300 parody best. People usually say Casa bonita or scott tenorman must die.
My favorite are: 1.) the special ed summer camp where they follow towellys life, documentary style. 2.) war on imagination trilogy 3.) when cartman is trapped in ice and goes to the future for a Nintendo.
Might have to agree with the Asspen one. There's so many that its tough to pick just one, but I also love all the Butters-Cartman centric episodes.
When Cartmen thinks he is dead and gets butters to help him atone for his sins to make things right so he can go to heaven. Hilarious every time.
Agree on Scott Tenorman Must Die, Casa Bonita, and Make Love Not Warcraft. I'd also like to acknowledge Woodland Critters Christmas, and Christian Hard Rock (where Cartman forms Faith+1). And for "Classic" South Park, Starvin' Marvin, Rainforest Schmainforest, Chinpokomon, and Cartman Joins NAMBLA.