Return of the Lord of the Rings To The Two Towers Video
Trapped In The Closet
Cock Magic
Skank Hunt
Le Petit Tourette
So many could be on this list though:
The F Word, which is the one about the bikers.
Good Times With Weapons, where the kids take on anime personas.
Grounded Vindaloop, which made me almost piss myself with laughter at Cartman instructing Butters in "VR."
Cartmanland
Medicinal Fried Chicken
A Song of Ass and Fire/Titties and Dragons
Cartman Joins NAMBLA
Imaginationland
The episode where everyone pisses in the waterpark, causing a piss tsunami
The episode where there's an internet outage so everyone migrates to "Californee" to get internet, based on rumors that they still have it
Chinkpokomon
The episode in which Randy takes a record-breaking **** and registers it with the European Fecal Standards and Measurements Institute in Zurich, only for Bono to beat his record
So many good ones, and they're still doing it after 25 years.