In the Arrow episode "Suicide Squad", Harley Quinn makes a five-second cameo, with a line supplied by Tara Strong herself. It is probably the most memorable part of the whole episode.
"Sebastian" on Babylon 5 — only in one episode, arguably the most memorable character in the whole series. Even if he's remembered, not as a hero, not as a messenger, not even as Sebastian... but only as "Jack".
Morden in his early appearances. He just oozes intrigue and menace, and he's only on screen for about five minutes.
In the new Battlestar Galactica series, none of the Mauve Shirt Viper/Raptor pilots are more memorable then the "Tattooed Pilot" whose actually more of an extra since he plays no vital role and has only one speaking line in the entire series.
Racetrack has a bit of a following too, despite never having actually had her own storyline, she's probably survived more raptor mishaps than Athena and Boomer put together, especially in later seasons.
Lord Flashheart in Blackadder, especially in the second series where he appears out of nowhere, goes through his monologue, and disappears again in less than two minutes of screen time, and totally runs away with the episode.And the girl. "Woof!"
In an interviewRik Mayall said he only agreed to play Flashheart if he got more laughs than the main character in that episode.
Peter Cook as Richard III in the first ever episode of Blackadder.
Stephen Fry too, in Blackadder the Third as Wellington, to the point that his mannerisms were carried over to General Melchet's character in Blackadder Goes Forth.
"The men had a whip-round and got you this... well, what I mean is that I had the men roundly whipped until they got you this. It's a cigarillo case engraved with the regimental crest of two crossed dead Frenchmen, emblazoned on a mound-of-dead-Frenchmen motif."
Tom Baker as Captain Rum. "Arr..." (Although in the last two cases it's more of a One Episode Wonder.)
Denis Lill as Sir Talbot Buxomley in Blackadder III episode "Dish and Dishonesty". He appears for about two minutes and dies at the end of his scene. But he is absolutely unforgettable.
Danny Trejo on Breaking Bad as Tortuga, the cartel snitch. Technically in for two scenes, the second one being somewhat more memorable.
The junkyard owner who saves Walt and Jesse from discovery by Hank in the third season and the arms dealer who sells Walt his gun in the fourth season also count.
The Buffy the Vampire Slayer episode "Conversations With Dead People" features a very entertaining Warrior Therapist vampire who died at the very end of the episode. According to the DVD Commentary for the episode, Jonathan M. Woodward's performance as said vampire was such a scene stealer that he subsequently landed larger roles on Angel and Firefly.
Then there's producer David Fury's 15-second appearance in the musical episode "Once More With Feeling", singing about the dry cleaner.
"They got the mustard... ouuuuuuuuuuuuuuuuuuuut!"
And then there was the Cheese Man from Restless.
The Castle episode "Overkill" somehow manages to have two One Scene Wonders within ten minutes of each other, in the form of Stephen Full as Benny, a charmingly sleazy and hungover motel clerk, and Jennifer Hall as Rebecca, a weepy lab technician who's 'cry-talk' Beckett has to decipher.
Drug dealer Vulcan Simmons appears in a three-minute scene in episode 3-13 "Knockdown" and isn't even guilty of the crime they believe he committed. He still manages to establish himself as a monster just by talking. See here. He'd be brought back in Season 6 and killed off in that season.
Colbert said in an interview that they also wanted to lampshade the absurdity of having gotten Henry Kissinger to oversee a guitar-playing contest by having him say, "Where are my pancakes? I was promised pancakes." But he wouldn't, and according to Colbert, somewhere there exist several minutes of footage of him begging Kissinger to say the pancake line.
Try to find a season of Criminal Minds that doesn't have at least one of these. The woman at the convenience store that grabs a shotgun in "Identity" is an example. The single-episode characters are usually so interesting that even the main actors have said in interviews that they wish they could guest star on their own show.
The concept has carried over to Spin-Off series Criminal Minds: Beyond Borders, usually in the form of the local authorities that the IRT is coordinating with. S1E7, "Citizens of the World", features a local policeman whose answer to everything is to beat someone up.
During the 1970s, Vega Nexos, a weird-looking alien killed off about five minutes into "The Monster of Peladon", received disproportionate fame in the fandom thanks to being added to make up the numbers to a series of breakfast cereal trading cards.
Cyril Luckham (the White Guardian) of the "Key to Time" storyline. He does nothing but sit in a chair and set up the plot for the season in the first five minutes of the first episode, but eerily sticks in your mind.
The Doctor: You mean nothing will happen to me? White Guardian: Nothing at all. Ever.
The Raston Robot Warrior in "The Five Doctors", which makes the most of its screen-time by slaughtering a horde of Cybermen. (It's even better for those who regard the eighties Cybermen as a Dork Age.)
In-Universe example in Drake & Josh. In the episode "Theater Thug", while Josh portrays the eponymous character during a film at the movie theater, Drake portrays a typical bystander with just one line ("Whoa! Just take it easy, man!"), which gets him popular with the other girls just for that one line.
The Robert de Niro episode qualifies as this, since the sheer amazingness of having de Niro as one of the celebrity characters is lampshaded with a lot of gushing about how amazing it is that Andy is going to meet Robert de Niro, and then subverted when he decides not to. In the end he's only in the show for a minute, and spends that minute being inordinately amused by a novelty pornographic pen. Needless to say, it's one of the most memorable guest spots of the series.
The George Michael scene from the Christmas special probably also qualifies. Although Michael is probably the biggest name in that show, he just wanders unexpectedly into the scene without any fanfare and not in his capacity as a celebrity, to deliver a hilarious performance centering on his own reputation for getting arrested for having sex in public places, which is simultaneously played straight (he drops by the "queer bench" in the park to ask if there's "any action") and subverted (he does this during his lunch break while on community service, which he's been sentenced to for... helping a fellow celebrity illegally dispose of a fridge-freezer).
Patrick Stewart appears for a single scene, in which he explains a screenplay he's writing wherein he plays a Professor Xavier-type character who uses his powers to make women's clothes fall off.
Dame Diana Rigg who is in two scenes. In the first she gets a condom flicked onto her head while she's eating soup, and instructs the perpetrator (Daniel Radcliffe!) on how to politely ask for it back again. In the second she wearily tells him to go away because he's been trying to hit on her all day).
Jubal Early in Firefly, who only appeared in the last episode yet was perhaps the greatest part of an already great series.
Frasier had John Glover make a guest appearance as a high-powered executive at the radio station. And It. Is. Glorious. The scene managed to earn Glover an Emmy nomination.
Both Friends and, more recently, Skins make a habit of doing this for the main characters' parents. Chandler's dad steals most of his scenes, Joey's mum likewise for her single appearance, and perhaps the most memorable scene that had Hugh Laurie lecturing Rachel on the plane to Britain; meanwhile Skins had cameos from Harry Enfield, Bill Bailey, Peter Capaldi, Arabella Weir, Josie Lawrence...
In the season one finale of Fringe the fact that Olivia was in a parallel universe where the Twin Towers are still standing was heavily overshadowed by the fact that she'd just met the mysterious William Bell, played by Leonard Nimoy.
Nimoy as William Bell deserves some sort of minimalist record for this. He had two lines in the Season 1 finale, then showed up for less than a minute in a fragmented flashback to the same scene four episodes later, and then had another one-line cameo in the mid-season cliffhanger. Then, the character was conspicuously absent in the Whole Episode Flashback "Peter", with a lame excuse about being away on business, and then Nimoy retired from acting after filming one more appearance for season 2.
Game of Thrones: David Rintour shows up for all of five seconds, in a flashback as the Mad King, but he owns those five seconds, as he utters the three most iconic words in the Game Of Thrones backstory:
"Burn them all! BURN THEM ALL!"
Pretty much any show where Summer Glau ends up being cast as a bit character ends up with her stealing the spotlight in her scenes. A good example is the Angel episode "Waiting in the Wings".
She appears As Herself on a train in The Big Bang Theory, where the characters come up and try to hit on her one at a time. Her reactions to their awkward advances are hilarious.
In the Dollhouse episode "The Left Hand" (2x06), there is supposedly some other plot involving Echo and Senator Perrin, but it's hard to pay attention to that when you've got Summer Glau and Fran Kranz on the same screen together.
On Glee, one of the most memorable one-scene wonders is Cameo, an unruly student from a flashback to Holly's past that explained why she became a free spirit.
Cameo: Tricks? You some kind of magician substitute? I'm a Christian, and that devil magic stuff OFFENDS ME!! (charges Holly and punches her lights out)
Holly:(in the present) I woke up to an empty classroom. And worse, they took my Air Jordans!
Seth Green and Breckin Meyer in Heroes, as a pair of comic book store employees who have the "best day ever" when they briefly get to help one of their favorite characters.
Heroes also has John Glover as Samson Gray, Sylar's real father.
Not exactly a One Scene Wonder, but in the fourth series of Jonathan Creek Adrian Edmondson turns up about once an episode and manages to steal every scene he's in as Carla's clueless, pretentious but strangely lovable producer/husband Brendan.
Strangely lovable because he's so incredibly easygoing, not even blinking when Carla makes out with Jonathan (with full-on tongue action) right in front of him. Perhaps his best moment was admitting he'd once been married to a man in the US. But it was only a marriage of convenience! And they never had a co-production deal, because that would just be wrong.
Adam Arkin as Theo Tonin, The Don of the Detroit Mob in Justified. Reputed to talk to a severed human ear when he's angry.
Brian Cox as Vesper Abaddon in Kings. A deposed tyrant, he has two scenes where he tries to make his captor, Silas, as vile as he is, and another where he speaks to The Hero, David, before he is to be executed. Scary as hell, incredibly complex, and has only a few minutes screentime.
A 1994 episode of The Late Show With David Letterman had Dave ask "Johnny Carson" to deliver the Top Ten list. Larry "Bud" Melman delivered it posing as him. Then Dave said there was something wrong and this wasn't the list and called for "Johnny" again ... and out steps the real Johnny Carson, to nearly three minutes of continuous standing ovation. He sat in Dave's chair, and left without saying a word. This turned out to be Johnny's last television appearance.
In the third-season Lois & Clark episode "Double Jeopardy", there's a seemingly throwaway scene where Luthor makes a back-alley deal with a rogue government agent. It's amazing.
Whenever Charles Widmore is in an episode of Lost, he usually only has one scene, but that scene is always a killer.
"The fact that she never received your sentiments is good for her, because as far as she's concerned you've forsaken her. And that's the way it's going to stay."
"You creep into my bedroom in the dead of night, like a rat, and have the audacity to pretend that you're the victim?"
"Walk with me, Desmond." (Cut to Desmond standing around awkwardly while Widmore uses a urinal)
"One sip of [McCutcheon whiskey] is worth more than you can make in a month. What you are not, Mr. Hume, is worthy of drinking my whiskey. How can you ever be worthy of marrying my daughter?"
His daughter Penny also qualifies. She shows up even less often than her dad, rarely has more than one or two scenes, but they're always important, and the intensity of her love for Desmond always shines through, so much so that she and Des are among the most popular couples in the entire show, despite their limited screen time together.
Merlin had the Fisher King, an ageless old king who has waited years for Merlin to arrive and release him from his eternal life. The actor (Donald Sumpter from Game of Thrones) infuses the character with so much gravitas and poignancy that he turns a single scene into a bona-fide tear-jerker. The lighting and music only adds to the epic nature of the scene.
In Mighty Morphin' Power Rangers, King Sphinx, a Monster of the Week, only appeared in one episode, and was never seen again in the franchise. Despite this, he was a favorite character in merchadise related to the show, such as toys, coloring books, posters, and sticker albums. (Supposedly, the character was supposed to make a return appearance, or possibly more than one, but Word of God claims that the monster suit had been damaged too much to be used again.)
King Sphinx likely was given marketing prominence due to appearing in one of the original pilots, which is where many of the American products drew from (such as the outfits worn by the Ranger teens, used almost exclusively in the coloring books).
A minor controversy erupted when the Academy of Television Arts and Sciences served up an extreme version of this in 2006, nominating Ellen Burstyn for an Emmy for her 14-second, 38-word cameo in the TV movie Mrs Harris.
In "Valentine's Day", a second-season episode of The Office (US), Conan O'Brien appears in the background at Rockefeller Centre, as Michael is wandering around the streets of New York. Even funnier because Michael is watching a tall woman with glasses he thinks is Tina Fey.
The Blind Seer from Once Upon a Time, first appearing as a little girl and then a young woman over the course of a single episode. It's not only her stitched-up eyes or the fact that she has eye balls on the palms of her hands, but that she tells Rumplestiltskin a Self-Fulfilling Prophecy that pretty much kick-starts the plot of the entire show.
Zelena's adoptive mother is one simply because she is a loving, and above all normal, person. This helps make Zelena's backstory very realistic.
Mr. Flibble, a penguin hand-puppet, visibly played and voiced by one of the regular cast during the last scenes of a single episode, never to be seen or even mentioned in the show again... but he's one of the most popular supporting characters, almost to the point that you could consider him the show's mascot, and he's got his own section on the show's official website, where he acts as an interview host.
Lister's friend Petersen, who has appeared in only two episodes ("The End" and "Stasis Leak") plus a few flashbacks in the first two series is nevertheless one of the most liked characters in the series. The fact he wasn't able to be brought back for Series 8 is a serious What Could Have Been for most fans.
Robin of Sherwood: Every fan talks about John Rhys-Davies' performance as King Richard. He was in exactly one episode: "The King's Fool".
One skit had an interview with Wilson the volleyball from Cast Away When the skit ended, Wilson said his ride was here and Tom Hanks casually walked onto the set. He didn't say anything, he just walked onset, stood there for two minutes waiting for the cheering to die down, picked up Wilson, and left. That is a One Scene Wonder.
Another Tom Hanks example was his surprise appearance (As Himself) on Celebrity Jeopardy!. He wasn't hosting that night (Will Ferrell was), but he came on to be one of the contestants. To say he blew the scene out of the water might have been an understatement. From getting his hand stuck in a pickle jar to suffocating in a plastic bag to banging his head off of the podium and breaking it, let's just say SNL might need a new Jeopardy set; Tom Hanks is all done chewing it to bits.
Steve Martin has made it a habit of just dropping in with no warning, often sending himself up as a major egotistical star and the crowd goes nuts every time. Once, Martin just showed up in the middle of "Weekend Update," saying he wasn't there to plug a movie or anything. "I just felt like a cameo."
It is also a virtual certainty that if the show has any recurring feature that pokes fun at a specific actor or political figure the person being mocked will eventually show up in the middle of the feature and completely steal the scene. For example, one open had Tina Fey as Sarah Palin doing a press conference. In the middle of it, cut to back stage, where Lorne Michaels is talking to the real Sarah Palin about the skit...and then Alec Baldwin comes up and mistakes Palin for Fey.
Prior to Palin's appearance, the most memorable example was probably the time Janet Reno showed up unannounced to take over a "Janet Reno Dance Party" sketch from Will Ferrell doing a Reno impersonation.
For most of 1995-96, a recurring sketch would be "The Joe Pesci Show" with Jim Breuer as a Pesci who would talk like his movie characters, get nuts and hit guys with a bat. Colin Quinn played Robert De Niro as if it was still the 1970's and talk in "one word sentences." It all ended with the real Pesci and De Niro coming on to beat the two up.
Jimmy Fallon would host the show in 2014, with musical guest Justin Timberlake; the two naturally would participate in a Barry Gibb Talk Show sketch. At the ending theme of the sketch, the real Barry Gibb would sing the falsetto parts on the "Nights On Broadway"-soundalike theme song alongside Fallon and Timberlake.
Not exactly a one scene wonder, but Christopher Walken probably does deserve credit for managing to completely steal the show every time he hosts.
When the real David Patterson appeared alongside Fred Armisen's impersonation. He criticizes the show for making fun of the blind...then proceeds to act the same as Armisen did
The What Up With That faux-talk show/musical variety that Kenan Thompson hosts frequently has two special guests cameos of real celebrities playing themselves as if they were on a normal talk show. Those cameos are played straight and frequently steal the scene (though Jason Sudeikis often steals it back). Cameos include Al Gore, Mike Tyson, Morgan Freeman, Robin Williams, Bill O'Reilly, and Samuel L. Jackson, who used profanity on the live show.
Once an Episode, one of the impersonated celebrities on What's Up With That is Fleetwood Mac guitarist Lindsay Buckingham, as played by Bill Hader. During one WUWT sketch, the real Lindsay Buckingham shows up in the end singing the theme song with the sketch participants and Hader!Lindsay, and play guitar.
Susan's parents in Seinfeld are far more interesting than they have any right to be. Her father because of his affair with John Cheever, and his mother because of her downbeat caustic attitude during her first appearance. ("If I had a nickel for every one (of the books in the library) he's actually read... I'd be broke.")
Mr Bookman, the library cop from "The Library".
Mike Stamford in Sherlock. Molly was supposed to be in one episode but England loved her so much, they expanded her role.
David Rees Snell, who played Ascended Extra Ronnie Gardocki on The Shield, played season four Big Bad Leon Drake, an evil Cobra Commander-type terrorist who was the Big Bad for the fourth and final season of The Unit. Despite appearing in only seven episodes (with most of those episodes featuring him in one or two scenes, barking orders to his army of minions from his secret lair), David Snell's role is largely the only notable thing about the show in the eyes of fans of Shawn Ryan's other, more famous show The Shield.
David Rees Snell pulled this off again with his role as the mysterious Navy Seal named Hopper in "Last Resort".
In episode 2x12, despite featuring major advancement on most of the season's main storylines and one character's Crowning Moment of Awesome, most of the online chatter about the episode centered around former The Shield star Kenny Johnson's surprise cameo as an out-of-town member of the Sons of Anarchy biker gang summoned to help participate in a planned rumble with a rival Aryan gang.
Another The Shield cast member Walton Goggins comes in later and steals the show as a transgender prostitute.
Then there's the scene where Stephen King comes in to dispose of a body and outshines everyone with only a few lines of dialogue.
A few examples in Star Trek: The Next Generation: Sonya Gomez, the famously bumbling junior engineer; Nick Locarno in "The First Duty" (so much so that they wanted to use that character in Voyager, but had to settle for just the actor); Robin Lefler (due to the fact that she was played by Ashley fricking' Judd); Commander Shelby in "The Best of Both Worlds"; Captain Jellico and Gul Madred (played by Ronny Cox and David Warner, respectively); etc.
Stephen Hawking's appearance as his holographic self in the pre-credits scene of "Descent: Part I". Not only was Stephen the first, and, thus far, the only person to appear As Himself in a Trek production, but hewins his game of poker against Data and fellow holograms Albert Einstein and Sir Isaac Newton.
In Star Trek: Deep Space Nine, there was the Klingon chef who owned a restaurant on DS9, and also played Klingon folks songs to customers on the accordion. (Actually appeared in two episodes, "Melora" and "Playing God"; novels claim his name is Kaga, possibly meant as Homage to Chairman Kaga from Iron Chef, which debuted the same month as "Melora".)
Supernatural has the fourth Horseman, Death, played by Julian Richings, who manages to exude pure awesome simply by being there, despite roughly six minutes of screen time.
Jamie MacDonald is only in a handful of scenes in The Thick of It (he doesn't even get his last name until The Movie) but manages to be one of the most memorable characters in a show full of memorable characters.
From the same series you also have Cal "The Fucker" Richards, who is only in half an episode but gets some of the best lines of the whole series.
The Tudors: Peter O'Toole as Pope Paul III only appears in some episodes, never interacts directly with the main cast (being as he is in Rome all the time), and completely steals the show. Peter O'Toole should play the Pope in anything that has a Pope.
Alyson Hannigan as Trina Echolls on Veronica Mars might qualify. She shows up completely unexpectedly and completely dominates the intro scene with herself, Kristen Bell and Jason Dohring.
And in a later episode she reunites with her Buffy the Vampire Slayer co-star Charisma Carpenter (Cordelia) playing Kendall Casablancas. Trina runs into Kendall after she just spent a night with Trina's brother Logan and they have a fun, catty conversation.
Comedy actor Guillermo Francella (without his trademark moustache) played the until then unseen Big Bad of Argentine telenovelaVidas Robadas: he appeared in three scenes in the last two episodes, and completely owned the show. To picture the impact of The Reveal, imagine: the Magnificent Bastard head of a human trafficking net is finally seen on camera - and it's a completely serious and creepy-looking, say, Adam Sandler.
President Bartlett was originally supposed to be a One-Scene Wonder on The West Wing; the show was meant to focus on the senior staff, with the President appearing maybe once a month or so to emphasize the distance between the man at the top and the people working for him out of the spotlight. But Martin Sheen's performance was made of too much awesome, and on top of this it was decided that it would be silly to have a series set among the senior staff of the White House where the audience never saw the President, and so he got promoted to a member of the main cast.
Roger Rees as Lord John Marbury isn't quite a One-Scene Wonder — he was in five episodes over six years — but you can't deny that he was disproportionately memorable in his few scenes as the Loveable Rogue.
Brother Mouzone in The Wire only appeared in six episodes, most of them for just a few minutes, or seconds in the case of his introduction. An erudite, soft spoken, Harpers magazine reading, suit and bow tie wearing gentleman... who also happens to be one of the most feared and respected hitmen on the east coast, and whose popularity rivals some regulars.
Maggie Scully (Scully's mom) has a surprisingly large fanbase, despite only appearing quite briefly in a handful of episodes. It probably has something to do with the fact that she's such a nice, reasonable, normal person, especially when you compare her to Mulder's family. And of course, she has to put up with a lot in the show, including her husband dying of a heart attack, her daughter being abducted by aliens and presumed dead, her other daughter being shot dead, and her son being a total douche.
Cassandra Spencer, played memorably by Veronica Cartwright, is so central to the show's mythos, it's hard to believe she's only in 4 episodes.
Peter Boyle as the one-off character Clyde Bruckman, in the episode ''Clyde Bruckman's Final Repose". One episode. Not an important episode. Not a character important to the myth arc. But he's one of the most memorable characters in the whole show, partly because it was one of the most highly-acclaimed episodes ever, and because...well... it's Peter Boyle.