South Park's success lies in the show's sharp wit mixed with potty humor. Matt Stone and Trey Parker's ability to lacerate pop culture, big issues and current events is also a big part of its popularity. It definitely doesn't need celebrity guests to carry it, but the inclusion of cameo performances is often a fun part of the show.
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Many cameos were so perfectly done that they didn't feel gratuitous at all, while some guest stars didn't add much to the show, remaining in the background. Fans have their favorites, but some guest stars were clearly more well-received and executed than others.
Best: Radiohead As Themselves
Many of the best celebrity cameos on South Park are held by musicians. Radiohead starred in one of the most brutal episodes ever, season five's "Scott Tenorman Must Die." It's such a South Park episode: they take a common trope, in this case, a bully playing a gross prank on Cartman, and turn it into an over-the-top revenge story.
When Cartman elaborately slays Scott's parents and feeds them to him in a chili cook-off, he adds to the humiliation and disgust of the situation by ensuring that Radiohead is there to watch. They mock Tenorman and call him a crybaby while he sobs over his parents.
Worst: George Clooney As Sparky
In season one's fourth episode, "Big Gay Al's Big Gay Boat Ride," George Clooney lends his vocal talents to... Sparky, the Marsh family dog. While the episode began as an offensive foray into dog behavior, it ended with more tolerance in a lesson about gay acceptance, even if it was still questionably delivered. It was a milestone for Comedy Central, but Sparky's voice, which was just a few barks, really wasn't all that impressive or important and had we not known, we never would have guessed Clooney was even involved.
Clooney was much funnier in South Park: Bigger, Longer & Uncut as Dr. Gouache, a parody of his ER character, Dr. Douglas Ross. He replaced Kenny's heart with a potato, giving him yet another death and proving what an incompetent doctor Gouache really was. So they blamed Canada, of course.
Best: Ozzy Osbourne As Himself
In season two's 14th episode, "Chef Aid," Ozzy Osbourne joined several other performers, including Ween, Primus, Elton John, Rancid, Devo, Rick James, DMX, Joe Strummer, and Meat Loaf, as guest stars in a concert to benefit the boys' beloved Chef. The benefit itself was funny, including Chef's own part in the fundraising, but Ozzy clearly had the best cameo.
Ozzy poked fun at his own trademark speech, claiming that he and Chef went way back, when Chef told him to "buy a pompadour hat" and he mistook it for "bite the head off a bat." He then bit Kenny's head off, giving the kid one of his best ever deaths.
Worst: Jay Leno As Mr. Kitty K
In season one, episode 13 dealt with Cartman's mother and her personal life, which of course comes into play again later in the show. Jay Leno gave a lackluster and forgettable appearance as Cartman's cat, Mr. Kitty. Like Clooney, Leno is sort of funny here because he's a big name giving vocal talent to a few "mews," but if you didn't pay attention to the casting you wouldn't even know he's in the episode.
Leno was much better as himself in "City on the Edge of Forever," in which Mrs. Crabtree, the bus driver, unintentionally beats Carrot Top at his own game and becomes a comedic sensation.
Best: Robert Smith, Himself
One of the least embarrassing guest roles in the history of South Park went to Robert Smith of The Cure. Barbara Streisand fans may have not liked the first season's 12th episode, "Mecha-Streisand," but the hilarious episode broke both South Park and Comedy Central records.
Robert Smith plays the only beast in the Kaiju film parody who can defeat her, a giant moth who flings her into outer space. His fans loved the role, especially his young nieces and nephews, who consider it his highest achievement. As Smith leaves, with the sun setting in true heroic mode, Kyle calls out, "Disintegration is the best album ever!"
Worst: Richard Belzer As Loogie
In the first episode of season four, "The Tooth Fairy Tats 2000," Richard Belzer starred Loogie, the leader of an organized crime ring centered around lost teeth. The show took a funny take on the tooth fairy myth and how the minds of children work when it comes to money, but Belzer, a great comedian, could have easily been used in a more suitable setting for his talents, even if the tooth fairy could be considered a great conspiracy.
It's not even that Belzer was a terrible guest, but that there were so many guest roles on the show used in better capacities. Belzer would be a fantastic addition to the show as a regular who comes up with conspiracy theories.
Best: Malcolm McDowell, Narrator
In season four, episode 14, Parker and Stone decide to mock the classic novel Great Expectations with an episode called, "Pip." None of the regular boys were featured in the episode, which starred the titular character. Stone dubbed it one of their worst episodes, even though many fans appreciated the parody.
Malcolm McDowell was the narrator of the episode, but he was listed as "British Person." He was incredibly funny and did a beautiful job, but fans can't help but wish he'd also been cast in a more nefarious role in another episode of the show, given his previous villainous characters.
Worst: Norman Lear As Benjamin Franklin
Season seven's fourth episode, "I'm A Little Bit Country," was also the show's 100th episode, and it took on the Iraq War, pitting citizens in the town for and against it against one another to the point where they, too, are at war. Cartman meets Ben Franklin while in a coma and asks him his own opinion.
Franklin is voiced by Norman Lear, actor and founder of People for the American Way, and he diplomatically gave Cartman advice that pretty much sounded like having your cake and eating it, too, suggesting that America go to war but also allow protest, as both are necessary.
Best: Korn As Themselves
The best South Park Halloween episode, and one of the show's best episodes ever, "Korn's Groovy Pirate Ghost Mystery" dealt with a missing corpse used as a puppet, pirate ghosts (ghost pirates), and the best celebrities featured on the show: Korn!
The band was given enough lines to actually mean something in the episode, sure, but they were also fantastic lines that juxtaposed against Korn's nu metal image and performance on the show, making them sound like a wholesome game of super sleuths a la Scooby Doo. They even had a goofy creature sidekick, "Niblet," who played the part of Scooby.
Worst: Bill Hader As Farmer #2
Actor Bill Hader voiced a farmer in the seventh episode of South Park's 15th season, "You're Getting Old." The cynical episode was a good one, but Hader was a bit wasted as a farmer after Randy's underwear. It was funny, but Hader's such a comedic genius that he really needed a bigger role to work with.
Stone and Parker had to know that Hader had much more potential, because not only did he guest star several more times, but he gave a great performance in season 17's first episode, "Let Go, Let Gov," as Alec Baldwin, which is the honorable mention for the best of this list. He's been many other characters since.