There are two South Park Halloween episodes worthy of note:
"Pink Eye," from the first season, is the best of the lot, featuring all of the absurdity and toilet humor for which we love the show. The plot revolves around a certain brand of Worcestershire Sauce ("it just makes everything taste so...British!") turning people into zombies, which the doctors diagnose as pink eye.
Perhaps even better is "Korn's Groozy Pirate Ghost Mystery" from a few seasons later, in which the kids endeavor to scare the other kids on Halloween using Kyle's grandmother's corpse. When the corpse is found to be missing after they dig her up, the police graphically tell the parents that it was probably necrophiles. Meanwhile, pirate ghosts are seen around town.
So along comes the band Korn, in an obvious Scooby Doo homage, to solve the mystery and save the day!