Buzz·Posted on May 26, 201320 Best Celebrity Guest Stars On FriendsLet the countdown begin.by Spencer AlthouseBuzzFeed StaffFacebookPinterestTwitterMailLink 20. John Stamos Season 9, Episode 22: The One with the DonorJohn Stamos guest stars as Zack, an unsuspecting sperm donor for Chandler and Monica. Zack is unknowlingly invited to dinner as a test to see if he will be a fitting donor. The couple scares him away, though, because of all the random health-related questions. This marks the turning point for Chandler and Monica, and they ultimately decide to adopt a baby instead. 19. Jim Rash View this video on YouTube Season 10, Episode 19: The Last One: Part 1Season 10, Episode 20: The Last One: Part 2Academy Award winner Jim Rash is only credited as 'nervous male passenger' in the final two episodes of Friends, but his one-liners make the character unforgettable. In the episode, Phoebe tries to contact Rachel before her flight for Paris departs. In order to buy some time, Phoebe claims that there is a problem with the plane's left phalange. Jim Rash's character overhears the phone conversation and abandons the plane, claiming: "oh, my god! This plane doesn't even have a phalange!" 18. Ben Stiller Season 3, Episode 22: The One with the ScreamerRachel brings a date (Ben Stiller) to the opening of Joey's play, but he turns out to be extremely short-tempered and yells at everyone for the slightest inconveniences. 17. Robin Williams and Billy Crystal Season 3, Episode 24: The One with the Ultimate Fighting ChampionRobin Williams and Billy Crystal burst onto screen by interrupting a conversation with the six friends, insisting that they make room for the two guest stars because there are no other open seats in Central Perk. Robin Williams' character mentions that he suspects his wife is cheating on him. The friends soon learn that Billy Crystal's character is the one who is sleeping with her. 16. Sean Penn Season 8, Episode 6: The One with the Halloween PartySeason 8, Episode 7: The One with the StainEric (Sean Penn) is originally engaged to Ursula, Phoebe's identical twin sister. Phoebe learns that Ursula lies in order to keep their relationship going, so she reveals this information to Eric. Eric and Phoebe develop feelings for one another, but Eric sleeps with Ursula, thinking that she is really Phoebe. 15. Brooke Shields Season 2, Episode 12: The One After the Superbowl: Part 1Brooke Shields (Erika) plays a crazed fan of Dr. Drake Ramore, Joey's soap opera character. Erika sends Joey fan-mail, and the two eventually meet for dinner. However, Erika believes that Joey is actually Dr. Drake Ramore, and after a few glasses of wine, she also starts to believe that the two are engaged. 14. Dakota Fanning Season 10, Episode 14: The One with Princess ConsuelaChandler and Monica take Joey to see their new house, but Joey fears that he will lose them as friends when they move. Mackenzie (Dakota Fanning) ultimately helps Joey realize that Chandler and Monica will be happier in their new house. 13. Dick Clark View this video on YouTube Season 6, Episode 10: The One with the RoutineThough Dick Clark never actually appeared in this episode, his name was mentioned several times, resulting in one of the greatest "Friends" scenes ever. Ross and Monica appear as background dancers for Dick Clark's New Years Eve TV broadcast, but they want more screen time on camera, so they perform their childhood dance routine. 12. George Clooney and Noah Wyle Season 1, Episode 17: The One with Two Parts (2)In efforts to boost ratings, ER doctors George Clooney and Noah Wyle joined the cast of Friends for an episode, fittingly playing another set of doctors. When Rachel Green sprains her ankle, she changes her name at the hospital in order to use Monica's health insurance. The studly doctors ask Monica (Rachel) and Rachel (Monica) on a date, and, in true sitcom-y fashion, hijinks ensue. 11. Christina Applegate Season 9, Episode 8: The One with Rachel's Other SisterSeason 10, Episode 5: The One Where Rachel's Sister BabysitsIt's a shame there wasn't an episode where all three Green sisters (Jennifer Aniston, Reese Witherspoon, and Christina Applegate) get to interact with each other. Still, Applegate held her own. Applegate's character gets Emma's ears pierced when she babysits for Rachel. 10. Alec Baldwin Season 8, Episode 17: The One with the Tea LeavesSeason 8, Episode 18: The One in MassapequaAlec Baldwin plays the extremely enthusiastic Parker, a man who Phoebe meets at the Gellars' 35th wedding anniversary party. Parker compliments everyone and everything, but this becomes annoying to the partygoers. 9. Julia Roberts Season 2, Episode Episode 13: The One After the Superbowl: Part 2Julia Roberts, aka Susie "Underpants" Moss, was a childhood friend of Chandler's. Year's ago, Chandler pulled down Susie's pants in front of the whole school, thus revealing her underwear. Viewing their reunion as the perfect opportunity for revenge, Susie goes on a date with Chandler and tricks him into wearing her underwear, leaving him stranded without any other clothes. 8. Jennifer Coolidge Season 10, Episode 3: The One with Ross' TanShe had sex with Evel Knievel and was a perfect arse. Jennifer Coolidge guest starred as the inauthentically posh Amanda, an older friend who Phoebe and Monica try to "cut out." Amanda reveals that Phoebe once tried to cut out Monica, resulting in a fight. Coolidge's fake British accent deliciously adds to the few scenes she inhabits. 7. Jason Alexander Season 7, Episode 13: The One Where Rosita DiesPhoebe takes a job as a telemarketer, and her first call is with Jason Alexander's character, a man who plans on killing himself. Feeling semi-responsible and newly attached, Phoebe tries to convince him otherwise. 6. Winona Ryder Tap to play GIF Tap to play GIF Season 7, Episode 20: The One with Rachel's KissWinona Ryder plays Rachel's sorority sister, Melissa, in one of the most memorable Friends episodes ever. Rachel admits that she and Melissa exchanged a kiss during a drunken night in college. Phoebe, however, doesn't believe this story, ultimately causing Rachel to kiss Winona's character again in an attempt to unleash information about their original kiss. Melissa looks too far into the new kiss and unveils that she loves Rachel. Awkward. 5. Paul Rudd Seasons 9 and 10Paul Rudd, aka Mike Hannigan, was first introduced as a random blind date for Phoebe. Mike later marries her - after an 18 episode stint throughout the series - claiming in his vows that she is "so wonderfully weird". 4. Tom Selleck Seasons 2, 3, 5, 6, and 9With a mustache so prominent that it made Chandler feel inferior, Tom Selleck guest starred as Dr. Richard Burke for 10 episodes on Friends. Richard was originally friends with Jack and Judy Gellar (Ross and Monica's parents), but he ultimately became lovers with Monica (his "chippie in the city"). Richard, already a father and grandfather, didn't want to have children, though, so he and Monica split. 3. Danny DeVito Tap to play GIF Tap to play GIF Season 10, Episode 11: The One Where the Stripper CriesRachel and Monica hire a last-minute stripper for Phoebe's bachelorette party, but he, Danny DeVito ends up crying. 2. Bruce Willis Tap to play GIF Tap to play GIF Season 6, Episodes 21, 22, and 23When Ross starts dating Elizabeth Stevens, his much younger student, her father (Bruce Willis) grows to dislike him. Eliabeth and Ross escape to her father's cabin, but Bruce Willis' character shows up with Rachel. In his Emmy-winning performance, Bruce Willis claims that he is a "neat guy" and starts singing and dancing to Love Machine, as a way of preparing himself for a night with Rachel. 1. Brad Pitt Tap to play GIF Tap to play GIF Season 8, Episode 9: The One with The RumorBrad Pitt guest starred as Will Colbert, Ross' best friend from high school. Will hated Rachel, claiming that his two biggest enemies are Rachel Green and complex carbohydrates, so he started a rumor that she was born with both male and female genitalia; he gave Rachel a teeny-weenie. This was especially rich because Brad Pitt and Jennifer Aniston were actually married when they filmed the episode.