It’s Always Sunny in Philadelphia has been running for almost two decades, and while the unlikeable lead characters running Paddy's Pub is the series' biggest appeal, so many Always Sunny guest stars have stolen the show. After 15 seasons, the It's Always Sunny in Philadelphia formula surprisingly hasn't gotten old, as the show has rarely strayed from what made it so successful in the first place, which is the so-called best friends and co-owners of Paddy's Pub constantly stabbing each other in the back for personal gain.
However, while It's Always Sunny in Philadelphia has an incredible main cast, including Glenn Howerton (Dennis), Charlie Day, (Charlie), Rob McElhenney (Mac), and Danny DeVito (Frank), It's Always Sunny guest stars have become some of the most lovable characters. The group of self-absorbed friends and family that run an Irish pub in South Philadelphia have often played second fiddle to some big-name actors. Some guest stars have reprised their roles multiple times because their characters are that good, but other guest appearances have left fans feeling they could’ve done without too. Nevertheless, the multitude of epic guest appearances is what has kept the series fresh, and the best guest stars of It's Always Sunny in Philadelphia have become iconic.
23 Retta
- “The Gang Gets Extreme: Home Makeover Edition”
Retta is best known for her role as Donna Meagle in Parks and Recreation — the girl knows how to treat herself, as evidenced by her famous meme. But her appearance on It’s Always Sunny was so small that it's easy to miss. In season 4, episode 12, “The Gang Gets Extreme: Home Makeover Edition,” Retta plays the clerk at the hardware store where the gang is picking up supplies. Just like so many other guest stars, Retta plays it straight and has to listen to Charlie's ludicrous ideas. The guest star is fun for fans of Parks and Recreation, but she doesn't have much material to work with.
22 Brian Huskey
- "2020: A Year in Review"
Brian Huskey features in season 15, episode 1 of It's Always Sunny in Philadelphia, "2020: A Year in Review." The episode has one of the most unique concepts in the series, as it takes advantage of all the catastrophic events in 2020 and suggests that the gang was heavily involved in all of them. This happens when Gary (Brian Huskey) audits the gang after the Paddy's Pub owners failed to repay several PPP loans, and Huskey's reactions to the gang's stories are priceless. Huskey appeared in Ant-Man, but his voice is more recognizable than anything else, as he's voiced various Bob's Burgers characters.
21 David Huddleston
- "A Very Sunny Christmas"
Fans went wild when show creators brought in the other Lebowski from The Big Lebowski (Huddleston also played one of the Johnston brothers in Blazing Saddles). Huddleston is best known for his role in the cult classic Coen brothers film as the billionaire that the Dude gets mistaken for, and his appearance in It's Always Sunny in Philadelphia might have become his second-best-known role. The comedy legend plays Frank’s old business partner in the DVD special "A Very Sunny Christmas," which sees him heavily involved in one of the gang's ridiculous schemes, as he tries to convince Frank he's a ghost before threatening to kill Frank.
20 Pablo Schreiber
- "A Very Sunny Christmas"
Pablo Schreiber is best known for playing George Mendez in Orange Is The New Black, who was overly creepy and even had the nickname "Pornstache." However, he was a lot less creepy during his appearance on It’s Always Sunny in which he played Ricky Falcone, a man whose toy robot had been stolen by Mac on Christmas, which also happens in "A Very Sunny Christmas." Mac felt guilty decades later and decided to return it, but he typically made Eddie's life worse and even sort of bullied him about his depressed lifestyle. Being one of the earlier episodes, Schreiber is one of the most underused It's Always Sunny guest stars.
19 Rob Thomas
- “Dennis Reynolds: An Erotic Life”
Rob Thomas' feature is one of the most random It's Always Sunny guest stars since he's a singer, not an actor, but the celebrity’s appearance during season 4, episode 9, “Dennis Reynolds: An Erotic Life” is one to remember — and it's one of the wildest casting decisions ever. However, considering the episode featured Dennis experiencing hallucinations and paranoia, it surprisingly makes sense. The musician has since revealed he got the gig, as Thomas invited the Always Sunny cast to a concert, which led to them offering him a part in the show.
18 Sinbad
- “Dennis Reynolds: An Erotic Life”
If Rob Thomas is one of the strangest It's Always Sunny guest stars, what's even stranger is who guested alongside him. Along with Thomas, Sinbad, a comedian and actor, who is best known for his role as Myron Larabee in the cult classic hilarious Arnold Schwarzenegger Christmas movie Jingle All the Way, featured in the same scenes. In the episode, Thomas and Sinbad play two patients in a mental institution who harass Dennis. Sinbad hilariously kept reminding everyone in the episode that Rob Thomas was the lead singer from Matchbox 20.
17 Geoffrey Owens
- "Frank's Pretty Woman," "The Gang Gets Invincible," "The Gang Makes Lethal Weapon 7"
Geoffrey Owens is one of the most interesting It's Always Sunny guest stars, as he has appeared three times, and he plays an actor who is hired by the gang each time. The actor has played "Fake Tiger Woods," "Fake Donovan McNabb," and "Fake Don Cheadle" in several of the gang's schemes, and each time, he doesn't look happy to be there and is simply picking up a paycheck, which makes it that much funnier. Though it isn't clear, Owens could very well be playing himself, as Mac and Dennis reference the fact that he starred in The Cosby Show, and in real life, Owens played Elvin in the sitcom.
16 Dax Shepard
- "Ass Kickers United: Mac and Charlie Join a Cult"
Dax Shepard has played some pretty weird characters in his career, such as Frito Pendejo in Idiocracy and Vince in Employee of the Month, so it only makes sense that he'd have the weirdest role of his filmography when he became an It's Always Sunny guest star. It could only have been a cameo in season 10, episode 10, "Ass Kickers United: Mac and Charlie Join a Cult," where he becomes one of Charlie and Mac's followers. Shepherd is great at playing dumb characters, and his role as a cult follower who religiously puts stickers on his face is the epitome of exactly that.
15 Dennis Haskins
- "Charlie Got Molested"
Dennis Haskins as the lovable principal Mr. Belding in Saved By the Bell, and he was also one of the earliest It's Always Sunny guest stars. During his stint in season 1, episode 7, "Charlie Got Molested," Haskins plays the gang's old teacher, who was accused of molesting children. At this point, Mac gets jealous and attempts to seduce the teacher by dressing in his old gym shorts. Haskin’s former co-star Patrick Thomas O’Brien has coincidentally also played a teacher in It's Always Sunny in Philadelphia season 2, episode 8, “The Gang Runs For Office.”
14 Christopher Lloyd
- "A Very Sunny Christmas"
Christopher Lloyd, who is an '80s icon and has starred in Back to the Future, Clue, Who Framed Roger Rabbit?, and recently joined Mandalorian season 3, has one of the best cameos of all of the It's Always Sunny guest stars, as even fans of Back to the Future don't recognize him. It's ironic that the most recognizable actor with the most prolific career of the best Always Sunny guest stars is so hidden that he's easy to miss. The actor was hidden behind a white Santa beard in the “Always Sunny Christmas Special.” In a flashback scene, he visits Charlie's mother on Christmas, and this memory causes Charlie to react violently when he realizes who Santa really is.
13 Josh Groban
- "Mac's Mon Burns Down Her House"
Josh Groban isn't just a great musician, but he proved in season 6, episode 6, "Mac's Mon Burns Down Her House" that he’s a great comedy actor too. Groban was cast as the short-lived boyfriend of Dee, though she eventually breaks up with him in favor of Brad Pitt during a daydream. Unfortunately, while It's Always Sunny has often leaned into its musical elements, while the sequence with Groban was funny, he was underutilized, and his musical talents could have been used for a Nightman sequel, a follow-up episode that has been in demand for so long.
12 Keegan-Michael Key
- "The Gang Goes on Family Fight"
Leave it to the gang to somehow end up on a Family Feud-inspired game show, which was aptly titled Family Fight. As hilarious as the concept is on its own in season 10, episode 8, "The Gang Goes on Family Fight," it was made all the better when Keegan-Michael Key was invited to play the part of the host. He interacts with the gang as they try to (and fail at) answer the trivia questions, and his part-parody of Steve Harvey along with mocking the Paddy's Pub gang makes Key easily one of the best It's Always Sunny guest stars.
11 Alexandra Daddario
- "Charlie and Dee Find Love"
Alexandra Daddario's role in It's Always Sunny in Philadelphia is hardly a guest star role, as the popular actor wasn't too famous when she appeared in season 4, episode 8, "Charlie and Dee Find Love." However, this early role shows how much of a screen presence she has. Before making waves in 2017's Baywatch remake and playing Rachel Patton in The White Lotus, Daddario briefly played Charlie’s love interest. To this day, Dadarrio’s character Ruby Taft is the only woman who’s made Charlie forget about his obsession with the waitress.
10 Stephen Collins
- "Dennis and Dee Get a New Dad," "Dennis and Dee's Mom Is Dead"
Stephen Collins made a name for himself as the Reverend and father-of-seven in 7th Heaven, which makes it ironic that he played Dee and Dennis' biological father in It's Always Sunny in Philadelphia, given how raunchy it is. Collins was featured in the season two finale where it was revealed he was the biological father of the twins, and he double-bluffed Dee and Frank when they were trying to scam him. Though Dee and Dennis were first swayed by his morally altruistic attitude, they soon realized that sort of life just isn't for them, and it's one of the very best It's Always Sunny guest appearances.
9 Jason Sudeikis
- "The Gang Gets A New Member"
Jason Sudeikis played the role of Schmitty in season 6, episode 8, “The Gang Gets A New Member.” His appearance was actually highly foreshadowed in the show. He was first mentioned in season 3, episode 3, “Dennis and Dee’s Mom Is Dead,” but he’s never shown on screen until the sixth season. Sudeikis reprises his character again in season 7, episode 13, “The High School Reunion, Part 2.” Sudeikis playing the annoyingly likable Schmitty is one of the most notable It's Always Sunny guest appearances, as Sudeikis has become one of the most popular TV actors thanks to the success of Ted Lasso, and he co-starred with Charlie Day in Horrible Bosses.
8 Oscar Nunez
- “The Gang Tries Desperately to Win An Award"
Oscar Nunez is TV sitcom royalty at this point thanks to his iconic role as Oscar in The Office. The actor makes a cameo in season 9, episode 3, “The Gang Tries Desperately to Win An Award" as the Sudz Manager. In the episode, the gang tries to improve the bar’s customer service after years of neglect, though Paddy's Pub ironically only ends up somehow getting a worse reputation. Nunez brilliantly plays into the clean-cut, by-the-book character he became knowns in the past two decades, and it's almost like a sitcom crossover and as if The Office's Oscar went on to become a bar manager.
7 Guillermo Del Toro
- "The Maureen Ponderosa Wedding Massacre."
It's only natural that Guillermo del Toro, the director of dark horror fantasy movies like Pan's Labyrinth and Hellboy, stars in the only It’s Always Sunny in Philadelphia episode that deals with zombies, season 8, episode 3, "The Maureen Ponderosa Wedding Massacre." del Toro played Pappy McPoyle in It's Always Sunny, and the episode is a horror parody where the gang crashes a wedding. Pappy initially tries to warn the gang about the looming zombie invasion, but his tendency to rant and speak gibberish makes him seem crazy (that is until the zombies actually did invade).
6 Sean “P. Diddy” Combs
- "Charlie's Mom Has Cancer"
Sean Combs played one of the most eccentric characters fans have seen on It's Always Sunny in Philadelphia, and he's one of the best It's Always Sunny guest stars. The rapper and music producer was cast as Dr. Jinx, a yellow-jumpsuit-wearing Doctor who ran his practice from his garage, in season 8, episode 6, "Charlie's Mom Has Cancer." The gang hired him to help Charlie’s mom beat cancer, though viewers were skeptical of his credentials. Combs has had a surprisingly strong career as a comedy actor, most notably playing the stern record label owner in Get Him to the Greek.
5 Judy Greer
- "The Aluminum Monster vs. Fatty Magoo"
Judy Greer has one of the most recognizable faces on TV, as she’s starred in almost everything. She’s most well known for her role as Kitty Sanchez in Arrested Development, but she’s also had stints on Californication, Royal Pains, Modern Family, House MD, ER, and Two and a Half Men. In It’s Always Sunny in Philadelphia season 3, episode 5, "The Aluminum Monster vs. Fatty Magoo," Greer plays an outcast from Dee’s days in high school, and they unsurprisingly lock horns when Dee learns Ingrid has become way more successful than her, and she's one of the best It's Always Sunny guest stars.
4 Dolph Lundgren
- "Thunder Gun 4: Maximum Cool"
Movie star Dolph Lundgren's It's Always Sunny guest role hilariously leans into his past as an action hero in macho movies. In season 14, episode, "Thunder Gun 4: Maximum Cool," the gang watches a test screening of the titular fictional movie, which is part of a franchise that has been spoken about so heavily in Always Sunny. However, for the first time ever, viewers actually get to see clips of a Thunder Gun movie, and Thunder Gun is played by none other than Lundgren. Lundgren is one of the best It's Always Sunny guest stars, as it's such a clever piece of stunt casting given what the actor's known for.