My two favs..
"How to eat with your butt" - where kenny get into his suit front ways for his school picture so his arse is where his head is, cartman finds it hilarious and sends it to a milk company as a 'lost child'. Then two people with a condition where their butts are where their head are try and find kenny as they think it's their son.
My other fave is one from the new series where cartman cant wait for a wii so he freezes himself.