Glenn Franklin Howerton III (born April 13, 1976) is an American actor of television and film. He is best-known for his role as Dennis Reynolds on FX's It's Always Sunny in Philadelphia (on which he is also a writer and executive producer). He also played Corey Howard on the short-lived 2002 sitcom That '80s Show. Description above from the Wikipedia article Glenn Franklin Howerton, licensed under CC-BY-SA, full list of contributors on Wikipedia.

Glenn Howerton plays as Dennis Reynolds in It's Always Sunny in Philadelphia.
Glenn Howerton is the top #1 member of the cast and appears in 161 episodes.

Robert Dale "Rob" McElhenney ( born April 14, 1977) is an American film and television actor best known for his role as Mac on the FX TV series It's Always Sunny in Philadelphia. He is also credited as creator, developer and executive producer of the show, which has also provided McElhenney with writing and directing credits on various episodes. He is married to Kaitlin Olson, who plays Deandra Reynolds on the show. He has also opened a bar in Philadelphia called Mac's Tavern. Description above from the Wikipedia article Rob McElhenney, licensed under CC-BY-SA, full list of contributors on Wikipedia.

Rob McElhenney plays as Ronald "Mac" McDonald in It's Always Sunny in Philadelphia.
Rob McElhenney is the top #2 member of the cast and appears in 161 episodes.

Charlie Day is an American television and film actor, from Middletown, Rhode Island, best known for playing Charlie Kelly on It's Always Sunny in Philadelphia. Besides playing Charlie Kelly on the hit FX television series It's Always Sunny in Philadelphia, he is also the show's executive producer and one of its writers. He has also appeared on several other television shows such as Third Watch, Law & Order, and Reno 911!. He appeared in New Line Cinema's Horrible Bosses in July 2011, opposite Jason Bateman, Jason Sudeikis, Kevin Spacey, Jennifer Aniston, Colin Farrell, and Jamie Foxx. He also had a role in the 2010 film Going the Distance, alongside Justin Long, Jason Sudeikis, and Drew Barrymore. In 2013, he voiced a character named Art in the Pixar animated film Monsters University and appeared in Guillermo del Toro's film Pacific Rim. Day hosted the November 5, 2011 episode of Saturday Night Live (SNL) with Maroon 5 as the musical guest. He was the second cast member from It's Always Sunny in Philadelphia to host SNL (after Danny DeVito, though DeVito hosted SNL before It's Always Sunny in Philadelphia premiered). DeVito made a special appearance during Day's opening monologue. Description above from the Wikipedia article Charlie Day, licensed under CC-BY-SA, full list of contributors on Wikipedia.

Charlie Day plays as Charlie Kelly in It's Always Sunny in Philadelphia.
Charlie Day is the top #3 member of the cast and appears in 161 episodes.

Kaitlin Willow Olson McElhenney (born August 18, 1975) is an American actress, comedian, and producer. She began her career in The Sunday Company at the Groundlings, an improvisational theatre and school in Los Angeles, California. She had minor roles in several television series before being cast as Deandra "Sweet Dee" Reynolds on the long-running FX comedy series It's Always Sunny in Philadelphia (2005–present). Olson starred as Mackenzie "Micky" Murphy in the Fox comedy series The Mick (2017–2018) and as Cricket Melfi in the Quibi comedy series Flipped (2020), which earned her a nomination for the Primetime Emmy Award for Outstanding Actress in a Short Form Comedy or Drama Series. She has also appeared in films including Leap Year (2010), The Heat (2013), Vacation (2015), Finding Dory (2016), and Arizona (2018). She has been married to actor Rob McElhenney, her co-star on It's Always Sunny in Philadelphia, since 2008.

Kaitlin Olson plays as Deandra Reynolds in It's Always Sunny in Philadelphia.
Kaitlin Olson is the top #4 member of the cast and appears in 161 episodes.

Danny DeVito (born November 17, 1944) is an American actor, comedian, director, and producer. He first gained prominence for his portrayal of Louie De Palma on Taxi, for which he won a Golden Globe and an Emmy. DeVito founded the production company Jersey Films with his wife Rhea Perlman.

Danny DeVito plays as Frank Reynolds in It's Always Sunny in Philadelphia.
Danny DeVito is the top #5 member of the cast and appears in 154 episodes.