I haven't even watched it for the last few years, but there are so many it's hard to choose. The original Mr Hankey of course. The Mr Mackey "drugs are bad mmmmkay?" episode. The episode where Cartman goes in search of his father, being conceived on the night of the "drunken barn dance". Ike gets circumcised. Mr Garrison invents a new mode of transportation, a gyroscope-powered monowheel "IT" that's controlled through a quite painful and unappealing method. Chef's chocolate "salty balls". The episode where they learn how to reverse the digestive process and cram food up their arse and poop out of their mouths. The one with the school nurse with the conjoined fetus growing out of her face.