1. The Death of Eric Cartman
Would South Park be the same without Cartman? If my number one favourite episode is anything to go by the answer is definitely no. The Death of Eric Cartman combines everything this show does best; the bully's malicious insanity, Kyle and Stan getting their own back, and Cartman's laugh-out-loud relationship with Butters. When Eric eats all the crispy skin off the KFC chicken before the other boys get a taste, the gang team up to ignore him completely. Failing to comprehend why anyone would simple ignore him, Cartman comes to the obvious conclusion that he is dead.
Of course, poor Butters is five steps behind and believes if Cartman is dead he must be able to see ghosts. If that wasn't funny enough, Butters's parents put him through a series of traumatising tests (anal probe, anyone?), Cartman decides to atone for his sins with gift baskets, and then he has to deal with a hostage situation. Plus, we get another rendition of "Lou, lou, lou, I've got some apples". We could watch it over and over again.