Santa Claus (aka Jolly Old Saint Nick, Chris Cringle, etc...). Santa first appeared in the infamous hand-animated, pre-South Park short "The Spirit of Christmas: Santa v. Jesus" done by Matt and Trey. He has since become a staple in the little mountain town. Santa was last seen on South Park fighting terrorists in "Red Sleigh Down" (617), and popping a cap in some Christmas Critters in "A Woodland Critter Christmas" (814). And also briefly in the bear mascot outfit in "201".
Season 7: Cancelled
The boys wake up in outer space.