Despite having been on the air for 28 seasons, South Park hasn’t racked up a ton of celebrity cameos over the years. Don’t get us wrong, some pretty famous people have lent their voices to the series, from George Clooney to the members of Korn, but the number isn’t anywhere close to that of, say, The Simpsons. Two of the more noteworthy—and quite frankly, impressive—guests that showrunners Trey Parker and Matt Stone were able to land were legend…