Our Miss Brooks: Miss Brooks often serves this role, as she is constantly dealing with the eccentricities of the people around her.
30 Rock: In one episode, Kenneth's page jacket is ruined by Jenna and the only way he can get a new one is by competing in a "page-off" against the head page Donny - a secret underground NBC trivia contest. Pete catches them doing it as none of the pages are upstairs working, and says "What, are you kidding me? This is a billion dollar company. Donny, give Kenneth a damn jacket!"
Liz thinks she's this but in truth, she's just as nuts and messed-up as the rest of the crew.
Alpha House: After he takes a hit in the polls for applauding something President Obama said in his State of the Union address, Sen. Robbert Bettencourt (R-PA) decides to play up this trope for his next primary debate. Given that his other opponents turn out to be A) a restaurant owner who boasts about his ability to dodge income taxes, B) a PETA treasurer, C) the self-proclaimed reincarnation of James Buchanan (who even debates in-character), D) an alchemist, E) a self-educated libertarian who advocates repealing all laws in existence, and F) a 9/11 truther, this is a pretty easy task for him.
The Andy Griffith Show: Sheriff Taylor is basically this for the whole town of Mayberry. Whenever he leaves town for any length of time, things tend to go haywire.
Another Period - as befits its Downton Abbey/Upstairs Downstairs dynamic - has two, both female. In the main household, eldest daughter Hortense Bellacourt fights to keep from being sucked into the craziness of her family, especially her younger spoiled-rich sisters Lillian and Beatrice. Meanwhile, new chambermaid Celine/"Chair" is struggling with similar lunacy in the servants' quarters.
Arrested Development: Michael Bluth believes he's this. He really, really isn't, but compared to the rest ofhis family he may as well be. In fact, he's probably only the third most sane person in his family, behind George Michael and Maeby. Funny enough, the true OSM is the narrator/documentary crew.
The A-Team: In one episode, Face confides to a client that he suspects Murdock is the only one of the Team who is actually in touch with reality, "...and the rest of us should just be sprinkled over ice cream."
The Big Bang Theory: Has two sane people in Penny and Leonard. Penny is there primarily to point out how hopelessly obsessed the geeks are, while Leonard is easily the most "normal" of the geeks. Leonard even frequently acknowledges what he and his friends are doing is completely insane, although he usually participates anyway. He does have his moments of insanity though such as in "The Itchy Brain Stimulation" which ironically has Sheldon at his sanest.
Big Time Rush: In one episode, James has sprayed himself with lots of orange spray, which is shown to them as "Hollywood Fever". Kendall, Carlos and Logan try to fix the situation, but Carlos ends up joining the Jennifers, and Logan becomes addicted to playing with bongos. Kendall is the only kid at the Palm Woods who manages to not change by this effect.
Edmund Blackadder. Surrounded as he is by idiotic and / or mad superiors and underlings. For all his faults, Edmund is usually the only sane person around at any given time period. Unfortunately for him, if there are any other sane people around, they're usually gunning directly for him. He's also a lot more savvy than most people around him in a historical sense as well, being able to recognize where his contemporaries cannot that witch hunts solve nothing, the application of leeches to various body parts isn't an effective medical treatment, and that World War I is one long, muddy, blood-soaked and pointless waste of time and lives.
"Whatever it was, I'm sure it was better than my plan to get out of here by pretending to be mad. I mean, who would have noticed another madman round here?"
In the first series, it is Baldrick, of all people, who has this role. He's more well-known as the Bumbling Sidekick he becomes in later series.
Blake's 7: In this circa 1980 Sci-Fi, the cowardly thief Vila (Restal) was the only crew member to habitually question "Why risk near certain death to make one point that the news will suppress when we could set up somewhere and let people come to us if they want to?" Early in their career, female characters sometimes expressed similar opinions but they soon became either unthinking fighters or equally unthinking back-up. A lowly Delta class in The Federation's strict regimen, Vila is the only man always known by first name like the women, though he claimed once that it cost him a lot of money to get Delta ID "Because I did not want to be a spaceship captain risking my neck for the Federation".
Blue Mountain State: Alex Moran and Craig Shilo appear to be the only truly normal/sane people at a football college full of bullies, stereotypical freshmen, party-goers etc.
The Bob Newhart Show: Hartley was one of these himself. For good reason, because he was a counselling psychologist and most of the other characters were his patients, and Bob did deal with several sane people.
Bones: Camille Saroyan zigzags between this and Da Chief. Clark Edison tends to act as this when he's in an episode.
Buffy the Vampire Slayer: This happens a few times, generally due to magic addling people's minds. One example is in season 5 where Glory is using magic to obscure the fact that she and Ben share the same body. Only Spike, who's not human, is immune to the magic, and tries to explain to the heroes what's going on: everyone else is unable to understand what he's saying, bizarrely misinterprets him or ignores or forgets immediately. After Spike tries to explain that Ben and Glory share a body, which all saw the proof of, others (affected by the spell) make statements like "So, we think Ben and Glory have a connection, but what kind?" eventually leading Spike to say in frustration, "Is everyone here very stoned?" Eventually Spike realizes why he is the only one who can remember that Ben and Glory share a body, but that is no help in getting the humans to comprehend him. In a non-magical example, Rona seemed to think she was the only one of this among everyone in Sunnydale.
Troy points out how the air conditioning unit takes themselves just a tad too seriously. When they discover one of their number killed their former leader, they prepare to lead him to a secret fridge but Troy yells "No! He killed someone! Take him to the police! What is wrong with you guys?"
Corner Gas: Has Lacey, although the title passes to Brent or Karen occasionally.
Cougar Town: Travis Cobb. Compared to the antics of his parents and their friends, the Only Sane Man is the 19-year-old socially awkward college freshman; however, he does occasionally indulge in their antics.
Steve and Susan each fill this role with their group of friends - beauty-obsessed age-fearing Sally, Cloudcuckoolander egomaniac Jane, brain-dead misogynist Patrick and, well... Jeff - but tend to trade the role back and forth during their own arguments. The point of the show is to contrast the two normal people (Steve and Susan) with the two incredibly insecure people (Jeff and Sally) and the two excessively confident people (Jane and Patrick). Steve and Susan's relationship isn't perfect and they themselves aren't perfect; they both have perfectly normal flaws and neither ignore them nor obsess about them (much).
Jane of all people fills the role in "Sex, Death and Nudity", calling out Steve on his attempt to shift the blame for his refusal to come to the funeral onto Susan (despite her not being in the conversation), correcting Patrick when he fails to get that the aunt she just introduced him is not the same as the aunt whose funeral it is, making an effort to clear the air with Susan, and expressing frustration with Steve when he gets an erection as a result of her talking about lesbianism.
Unfortunately, it all comes undone when she's the only one to succumb to the Giggle Loop.
The Daily Show: Portrays the studio host, Jon Stewart, as the Only Sane Man, with all his reporters being weirdos and lunatics with various obsessions and derangements. The Daily Show also significantly revolves around playing clips of journalists and politicians looking foolish, cultivating the impression that Stewart is the only sane man in the entire country. Jon Stewart has explicitly described The Daily Show's purpose being to point out the absurdities of government and the world. Since they focus on the crazy stuff (like pirates in the Indian Ocean), it's entirely logical that he looks sane comparatively. Sometimes, the roles are reversed. For example, a scene on the aforementioned Indian Ocean pirates has John Olivier trying to talk about it seriously, while Jon Stewart is making a bunch of pirate jokes and puns.
Degrassi: Hands off the hat of sanity to at least one character... usually. While one character has a zany poorly designed scheme, another character will be there to say "You're kidding, right?" The most consistent with the sanity hat are Jimmy, Ellie, and Clare. Jimmy is sane to offset Spinner's Attention span. Ellie paired off against Ashley, Marco, or Craig to point out when they were doing something stupid. Clare's paired off with Alli, who is on a mission to become popular, Clare gets to tell Alli when her popularity plans are dumb. Now, earlier I said most capable. All three of these examples have had off days, just not as many as the rest of the cast. Paige and Holly J have at times made this an Invoked Trope, suggesting they might be the only sane woman at the school. However they don't have the hat of sanity as often as the above. Like much of the tropes on TNG this has roots in the previous incarnations of the show. Snake was the sane man of the The Zits as was Heather for the twins, Voula for Stephanie and Lucy etc. etc.
Dharma and Greg: Greg. Greg's father, Edward, also counts. Although it should be mentioned he has long since resigned himself to the insanity that surrounds him. While not as overt as Greg, Dharma is sometimes the Only Sane One, when Greg's quirks (like his over-organizing) come up.
Doc Martin: Martin likes to paint himself as one of these, however Joan and her replacement Ruth have far better claims on this (albeit in different ways).
Poor Keeler in "The Seeds of Doom" spends the story acting like a sensible normal person dropped into a Doctor Who plot. He recognises that messing around with the seed pods is a really bad idea, fully expects everything to go horribly wrong, and doesn't understand why no-one tries to take him to the hospital when he ends up getting slowly turned into a Krynoid. When Sarah Jane attempts to treat him like a Victim of the Week he finally loses his temper and starts snapping at her that he's sick of no-one treating him seriously. One should ask why he was in such a toxic work environment in the first place... although the fact that his boss was in the habit of grinding unperforming employees into fertiliser perhaps explains why he didn't quit.
Rory could also be considered this. Given that both Amy and the Doctor have Cloudcuckoolander tendencies, Rory often has to act as the voice of reason. And not just in the Cloudcuckoolander sense either. Amy and the Doctor both have a tendency to abandon good sense altogether when they become too emotionally involved in a situation. Rory is easily the most emotionally stable of the three of them, making him the Only Sane Man in that regard as well.
Due South: Used very creatively to Lampshade a would-be case of The Other Darrin. After the actor who played Ray Vecchio left the show, the next episode features Benton Fraser completely dumbfounded at another guy that shows up claiming to be him, with everyone else at the station also insisting that this is Vecchio. Eventually it turns out Fraser was left out of the loop for a bit that the real Vecchio is undercover with the mob, and the new guy is an FBI agent who's impersonating him.
Eureka: Sheriff Jack Carter is one for the entire town, since it's entirely populated by Mad Scientists, and they predictably lack the common sense that would keep some of their experiments from going wrong and threatening the town. While Carter is certainly not as book smart as the rest of the town, including his own genius daughter, he's certainly practical enough on his feet that his quick thinking is always what saves the town from self-destructing from its own scientific curiosity.
John Crichton usually manages to keep his head while all those around him are losing theirs. Justified by the fact that he is the only human in the main cast and aliens are likely to react to situations much differently, as well as by the fact that most of the rest of the crew are rebellious escaped prisoners who the Peacekeepers locked up for the sake of maintaining strict order.
On the other hand, Crichton actually shows clear signs of mental instability as a result of his experiences over the show. He does manage to pull himself together and get things done, but his base level is pretty off what would be considered ordinary.
And if you've ever sat and thought about what his American pop culture references must sound like through the Translator Microbes, you have to come to the conclusion that he must seem very insane indeed.
This includes yelling in faux-Klingon at a new companion who is good at picking up languages.
Fraggle Rock: Boober frequently plays the role of Only Sane Fraggle; though not as often as he believes he does. The Minstrels have Murray as their Only Sane... Something.
Frasier: Frasier Crane thinks he's this trope — his psychiatric skills are always on overload, analyzing his acquaintances with a keen eye and all-too-aware that he's surrounded by his anxious, foppish brother, his happy-go-lucky slob of a father, his violent-tempered, sex-crazed best friend, and his Cloudcuckoolander of a housekeeper who doesn't understand Americans. What he doesn't realize is that when it comes to reacting to the problems and issues in their lives, they are all far, far more well-adjusted and sensible than he is. Also, Martin frequently receives this role in the plots. He's not faultless, but if Niles and Frasier are doing something stupid it's a pretty good bet that he'll step in with some advice that resolves things.
Friends: Very often had one character playing this for the rest of the gang. Earlier in the show the gang's Team Mom Monica normally held the position but less frequently post-Flanderization. Ironically Chandler, who was initially very immature seemed to take over: as the gang's Deadpan Snarker, he'd use the situations to set up one of his jokes. As the two of them eventually ended up together there was definitely a Designated Parents vibe set up. But there were plenty of occasions where they'd be the crazy ones and someone else would play the role instead. It usually depended on what joke the episode was going for as to who the Only Sane Man would be in any one situation.
Game of Thrones: Lord Renly Baratheon views himself as this after Ned Stark rejects his plan in "You Win or You Die." At the very least, he's a far more sensible person than his brothers or his "nephew" Joffrey.
The George Lopez Show: Angie is this to her family, comprised of her hard-nosed husband, dyslexic son, unstable daughter and alcoholic mother-in-law. She is also this for her side of the family, with a con artist brother and a spoiled niece; her father Vic even said she was the lucky one.
George himself also serves as one most of the time.
Green Acres: Oliver Douglas is either the most sane person, or the least—he's the only one who tries to view things logically, as it becomes increasingly apparent that in Hooterville, logic no longer applies.
Green Wing: Mac, Jake and Lyndon are the only sane people in the entire hospital, both getting dragged into the others' insane antics. Mac doesn't seem to mind any of this as he likes to play around with everybody else, though Sue is his limit but that's due to her stalking problem. Lyndon, on the other hand, can't stand any of it and gets more and more frustrated as it goes along. Jake doesn't get too caught up in it and is too chilled to be as perturbed as he might be.
The Grinder Nearly everyone in town is so star-struck by former actor Dean Sanderson, they ignore or accept his antics — except for his brother Stewart, who tries in vain to get anyone to realize that everything Dean says is nonsense. Stew's wife Debbie and co-worker Claire also seem to be immune to Dean's celebrity, but only for Stew is it a constant aggravation.
Halfway Across the Galaxy and Turn Left: X is a female example. Not only does she have to live with a gambling-addicted father, a scatterbrained mother, and a Cloudcuckoolander sister, she is also has to keep said family safe from the enemies it's made entirely on her own at around age 10 (a result of her literally alien culture). Also contains a subversion as her younger brother is a genius but otherwise perfectly ordinary for his age - yet X seems see him the same way the rest of her cuckoo family, mostly because his slight immaturity and genius causes her no end of problems.
Homeland: Subverted Trope: Carrie thinks she is this, because she's the only one who realizes that Brody is really working for Abu Nazir, about which she is, of course correct, but Carrie is also mentally ill, and not in a comedic way.
Carrie: I'm not the one who got it wrong, I'm the only one who got it right!
How I Met Your Mother: Ted is an unconventional case, as he's a bit ditzy and is often neurotic, impulsive, and irrational, but he constantly reverts to this trope whenever his friends start going off the deep end, is often told that he acts like the group's dad, and usually is the person everyone runs to when they've got a problem. One of his most impressive demonstrations of this is in "False Positive", when the other four all chuck their dreams and plans away and make selfish, shallow, cowardly decisions, prompting Ted to dish out a brutal tongue-lashing and literally forbids them from doing anything of the sort — they're going to do exactly what he tells them to do instead, or else. They are all extremely thankful for it afterwards.
iCarly: Freddie, complete with volunteering for a Liz Lemon Job (almost exactly Liz Lemon'sactual job, for that matter.) Also Carly early on but they seemed to move away from this so she could do more physical humor.
Jonathan Creek: Jonathan Creek already gets a mention under this trope's close relative Only Sane Employee, but for most of the fourth season he ends up being the only person involved in the production of "Eyes and Ears" (think Crimewatch with the production values of a Fox News chat show) who seems to care or indeed even notice how tasteless, factually inaccurate and generally awful it is.
In Kamen Rider Drive, the Special Investigation Unit consists of an eccentric chief obsessed with fortune telling, a hardheaded lieutenant who cannot pronounce the monsters' name right and is prone to overacting, an Otaku internet researcher who carries around his doll collection, a Mad Scientist, and a terminally lazy detective who moonlights as a monster-fighting hero. And then there's patrolwoman Kiriko Shijima, who despite being high-strung, is easily the most rational officer in the unit.
Kenan & Kel: Kenan Rockmore would qualify as he almost always shouts out "WHYYYYYY?!" whenever Kel does something wrong.
Kyle XY: Josh kept a detailed notebook on all the strange things that happen around Kyle, and concluded he was a space alien, which he constantly mentioned. Kyle (who was actually an escaped lab experiment) later had to admit that Josh was the person who came the closest to figuring it all out.
Law & Order: Special Victims Unit: FBI Special Agent George Huang, M.D. always sees the big picture, and is often the only reasonable person. If the rest of the cast ever actually listened to him, instead of using him to wheedle confessions out of perps, most of their problems would be solved.
After Huang's departure, this role was passed on to ADA Rafael Barba.
The League: Splits this trait between two characters, Pete and Kevin, who are seen as significantly more normal and well adjusted than their friends, and as such they are best friends.
The League of Gentlemen: Benjamin Denton is the only sane person in his plot thread on the show. Similarly, in every scene set in the Local Shop, whichever character Mark Gatiss is currently playing will be the only sane man. He will probably also be tortured to death.
Leverage: Subverted Trope. Nate believes he's the only sane man on the team because he's the only one who's not a thief. It quickly becomes apparent that he's an unstable, self-absorbed, risk-taking alcoholic and the rest of them, despite their law-breaking, are much closer to normal, or at least functional. He only starts getting better once he acknowledges that he's a thief too and no better than the rest of them. Eliot also seems to think he is, given how frustrated he often gets with the rest of the team's antics...but especially Hardison's.
LEXX: Kai, who, despite being a more-than-2000-years old, former-slave-of-his-worst-enemy, single survivor of an entire destroyed civilization as well as being dead still is the most rational, honest and intelligent member of his crew by far. Of course, the competition aboard Lexx is not what you'd call fierce....
Maid Marian and Her Merry Men: Marian is the only sane person amongst the heroes and the Sheriff is the only sane one amongst the villains. At times, it appears that this pair may be the only sane people in medieval Britain.
Malcolm in the Middle: the eponymous Malcolm to his delinquent, dysfunctional family, his impossibly nerdy, neurotic, and socially inept friends, his nasty, petty, stupid neighbors and classmates, and the entirety of the shallow, unappreciative, unfair society around him. Deconstructed, as being this trope for too long and going through too much frustration as a result of his sane-ness turns him cynical and bitter enough for his OSM-cred to start slipping in the last couple of seasons (thought the finale strongly hints that he earns it back once he dares to live up to his potential).
M*A*S*H: At one point, BJ suggests Klinger may actually be this. precisely because he's the only one actively trying to remove himself from the war.
Merlin: There is an interesting take on this. Merlin is the only one who notices the constant stream of assassins/monsters/enchantments/traitors that pop up all the time. You'd think that everyone else would get wise to it. It is then subverted when Gaius becomes the Only Sane Man by pointing out the ridiculous actions Merlin takes to take care of the problems, becoming no more sane than the people whose sanity he is annoyed with!
Monty Python's Flying Circus: The customer in the "Dead Parrot" sketch is another classic. Likewise the customer in the "Cheese Shop" sketch, who finally takes the only "reasonable" way out of the situation and shoots the shopkeeper. However, since this is Python and both characters are played by John Cleese, their sanity is relative.
The Muppet Show: Kermit The Frog qualifies as the Only Sane Amphibian, although he cheerfully admits at least once that he's only surrounded by crazies because he hired them. Contrast Sam the Eagle, who merely thought of himself as this.
My Family: Michael. Technically an Only Sane Boy, at least in the earlier seasons. His father Ben only thinks he is.
My Name Is Earl: By the halfway point of the show, Earl himself has become this. He even has a mini-Heroic B So D when he realises he is now the one sane and competent guy in Camden. This really comes into force as a major plot point in the prison-arc where Earl is the only sane guy in the entire prison, including the governor.
Newhart: Dick Loudon, where he dealt with eccentric, over-the-top characatures of people on a near-constant basis. George Utley (Tom Poston) is the nice but dim-witted handyman who is not too good at his job, but still insists that he'll be inducted into the handyman's Hall of Fame. Stephanie Vanderkellen (Julia Duffy) is the spoiled maid who also is virtually incompetent, while TV producer boyfriend Michael Harris (Peter Scolari) is the epitome of alliteration and yuppiness. There was Larry (William Sanderson) and his mute brothers, Darryl and Darryl, backwoods outdoorsmen who owned the Minuteman Cafe; Larry introduces the group the same way every time they make an appearance: "Hi, I'm Larry; this is my brother Darryl, and this is my other brother Darryl," and makes wild claims that often turn out to be true. Dick somehow tries to keep his sanity but finally snaps at the end of the final episode, "The Last Newhart" ... only for "Dick" to be the starring character in Dr. Robert Hartley's nightmare.
NewsRadio: Dave Nelson, but only sometimes. Occasionally he's completely crazy, and Lisa is the Only Sane Woman.
In the US version, Oscar labels himself, Toby, Jim, and Pam as Dunder Mifflin's 'Coalition of Reason'. Although it's not without a cost. Oscar seems perpetually on the verge of snapping.
While Jim is probably the most likely out of the entire Dunder Mifflin staff to point out his coworker's foibles and snark about them, he is more than willing to indulge them for his own amusement whenever he's bored, which is oftennote He's shown to be a skilled salesman who can exceed his sales quota with minimum effort. He'd probably be able to overtake Dwight if he cared more for the job.. In later episodes, though, Jim has shown that he's not immune from picking up the Idiot Ball, especially when he's put in charge of anythingnote Jim and Dwight cause complete chaos and destroy a good amount of inventory when put in charge of the warehouse, and he and Pam go through a good deal of drama in the final season due to their inability to communicate.
Ryan sometimes also took on this role, then he got promoted to corporate and his ego got the better of him, even when he got busted down to the office temp.
And more recently John Watson to the higher-functioning sociopath Sherlock Holmes in the BBC's Sherlock. Martin Freeman's made a career of it. Though, like in the books, he's also not weirded out by the guy.
Lestrade is this to Anderson and Donovan and, so is Sherlock. Heck, John, too. Why do they have to put up with them?
And John was the only one to realize Sherlock was in actual danger in the pilot, and the only medical person (other than Molly) to know that drugs are bad for your health.
Varies on Only Fools and Horses depending on the episode in question. Prior to the seventh season it was usually Rodney, though occasionally Uncle Albert would step into the role. Starting with the 1990 Christmas special "Rodney Come Home" however, Raquel would invariably prove to be the only fully sane member of the Trotter family.
Outnumbered: In this BBC comedy, the sensible Jake Brockman often ends up as this, when thrown together with his younger and crazier siblings Ben and Karen. (Resulting in endless frustration and eventual snarking on his part).
OnceUponATime: Henry (who is all of 12 years old) and Archie (Jiminy Cricket) are possibly the most sensible people on the show. Too bad no one bothers to listen to either of them. Emma also acts as this.
Pixelface: This is usually Claireparker's role. There might be some Fridge Brilliance at work here, as Claireparker comes from a survival horror game and therefore may be more grounded in reality and a have a more realistic idea of her own limitations than the other video game characters.
Power Rangers RPM: Has the villainess Tenaya 7, who once spent an entire episode asking the Big Bad if his evil plan had failed enough for her to go take a nap. There's also Ziggy, who is so Genre Savvy that you're surprised he doesn't look directly at the camera and inform the audience that yes, these people he hangs out with are serious. At one point, he actually does look directly at the camera and inform the audience that these people he hangs out with aren't Rangers, they just play them on TV; as a way to segue into a behind-the-scenes episode.
Red Dwarf: Lister, mainly because Rimmer and The Cat's decisions are too selfish and Kryten's are too limited.
The Red Green Show: Harold often tries to be the Voice of Wisdom trying to get his uncle and compatriots to see the foolishness of this week's Big Plan. After all, Harold does actually have a high-school knowledge of physics and other sciences, and so can usually predict how things are about to wrong, badly!
Reno 911!: Let us not forget Deputy Sven Jones and (most of the time) Jim Dangle. They are the only ones who aren't violent, racist idiots, or holier than thou religious zealots. Or conversely, no one but no one is sane on Reno 911!.
Revolution: Listen to any conversation where the group is discussing what to do next and you'll find that Miles Matheson is probably the one making the most sense. Now, if only Charlie would just listen to him. Also, Aaron Pittman has shown himself to have held on to his sanity and morals...and he has to watch people like Rachel going down the slippery slope into evil and be surrounded by people who never seem to listen to anything he has to say. "Kashmir" has Aaron being the first to figure out that the group was experiencing oxygen deprivation in the tunnels and managed to get everyone to realize that they had to get out and quickly.
The Saturday Night Live "Jeopardy" sketches LIVE by this trope.
In the second "Hip Hop Kids" sketch the kids are trapped in a cave and Andy Samburg's character tries in vain to convince the others that having dance contests against bears is a very bad idea!
Saturday Night Live pretty much lives off this trope. The vast majority of non-political SNL sketches are based on the concept of one or more sane characters interacting with one or more insane characters. The main variables are the ratio of sane characters to insane characters and the way in which the insane characters are insane. Often, crazy characters recur through a number of sketches while their sane foils are variable. For example, the "Two A-Holes" sketches follow this.
Scrubs: Like the case with most sitcoms, this role can be traded depending on what works for an episode. But the one who plays this on a regular basis is Carla, whom will often be the voice of reason, the only one in the hospital with any common sense and right solution to a problem. Because of this, she is the only one Perry Cox doesn't dare speak down to.
Seinfeld: Jerry Seinfeld once talked about how the character he played is actually funnier than the others because he "brings the sanity back" by making everyone else look just as crazy as they should in real life in comparison to him, and therefore even funnier than they otherwise would. This is especially true whenever another character tries to get advice from/explain whatever's going on to Jerry, who usually not only sees through, but plays along, often quite indulgently, with the other characters' bizarre motivations and logic. Also a Subverted Trope, as while Jerry thinks he's the only sane man, but it's more in comparison to his friends as he is shown to have a lot of troubles too, the most apparent being his tendency to break up for petty reasons, being a Neat Freak and being a bit of a Man Child.
Selfie: Henry Higgs plays with this trope. He's a grounded, very competent PR rep for a child's pharmaceutical company, and he chides Eliza for her overly self-indulgent nature, and often views touchy boss Sam Saperstein's actions with oddness. However, while being a fairly normal, polite, nice guy, Henry is also very into his work, and somewhat un-fun. The show seems to be getting across that everybody is at least little weird, in their own ways.
Shameless: Teenage Fiona is, for all intents and purposes, the caretaker of the family, and spends almost all her time looking after her siblings and her alcoholic father.
Sledge Hammer!. Dori Doreau is in an unenviable situation. Her boss, Captain Trunk, is popping pills to stay even marginally competent and is one raging shout away from a coronary. Her beat partner is the reason for Trunk's manic anger and depression. She is a normally competent human woman partnered with a borderline homicidal maniac. Who happens to be a cop. She sighs and gets on with it...
Soap: Benson usually played this role within the Tate household. Mary and Jodie took turns with the Sanity Ball in the Campbell household.
In Star Trek: Enterprise, two-part episode "In a Mirror, Darkly", Commander Archer is the only sane one of the bunch. He's the only one that has no personal ambitions, wanting to save the Empire from the rebellion. He also has more compassion than the other officers and enlisted men, being disgusted by their new invention, the agony booth. The background setting is a rebellion by the Vulcans, Tellarites, Andorians and other subject races is going to destroy the Empire in a matter of weeks. No hope. Archer gets wind of a ship the Tholians have "acquired" from the future. Despite the war being lost as it is, he can't convince anyone to go to the location and try to take the ship. Captain Forrest is far less intelligent than his Admiral Forrest counterpart in the real universe, being casually brutal. Forrest has no interest in a myth ship, somehow being anxious to throw his life away in a battle that is unwinnable. Even when Archer is successful in forcing the Enterprise to go to the area of Tholian space where the U.S.S. Defiant, Constitution Class is located, and the ship turns out to be everything that Archer was told, Forrest idiotically wants to destroy the ship, rather than take it back with them, insanely wanting to throw away any advantage the ship could give them, probably because it would make Archer look like a hero and Forrest's pride was bruised. Archer seems to be the only one that realizes that this ship could potentially win the war for them. When Archer manages to bring back the ship to his commanding officer, winning a battle against several vessels, the Admiral wants to waste several years taking the ship apart to figure it out. It's insanity: the war will be lost in a few weeks, tops. They have NO TIME to take the ship apart. Archer wants to use the ship to win the war. He's right: it's the Empire's only hope. The Admiral, like Captain Forrest, doesn't see the obvious. So, Archer ends up killing the Admiral in order to save the Empire. From start to finish, Commander Archer is the ONLY one who can see things as they are. He's the only one that isn't nuts.
However, he does suffer from hallucinations during part II.
Dr. Crusher in the episode "Remember Me", who is the only one to notice people disappearing. She really is out of touch with reality, but she's perfectly sane: it's the rest of "reality" which isn't.
Dr. Crusher: If there's nothing wrong with me, there must be something wrong with the universe.
In the Deep Space Nine episode "The Search Part II", Sisko, O'Brien, Dax, Bashir, and T'Rul are put through a Virtual Reality Mind Screw which posits a treaty between the Federation and the Dominion, an alliance that gives the Dominion the upper hand — and Bajor. These five are the only military officers (four Starfleet, one Romulan) on Deep Space Nine who see the treaty for what it is: a betrayal of Federation ideals, Bajor, and ultimately the Federation itself. Sisko turns down a bribe-promotion and our heroes risk vaporization or court-martial and a trip to Elba II to keep the Dominion away. Odo and Kira get them released and whew, it was All Just a Dream.
In the Star Trek: Voyager episode "Bliss", Seven of Nine, the Doctor and Naomi Wildman are the only ones who notice the crew acting strangely when they find an opportunity to get home. That was actually a telepathic pitcher plant making everyone hallucinate their own desires in an effort to eat them.
Specifically, the pitcher plant's powers make people consumed with their desires once they seem within reach. The Doctor, being a hologram, naturally isn't affected, and Seven and Naomi remain clearheaded because they don't share the others' intense desire to return to the Alpha Quadrant. Unfortunately, once Seven learns what's going on, she gets a desire to escape the pitcher plant, which itself makes her vulnerable to it.
Bobby is often the only one of the main four who can take a step back and see the bigger picture most of the time. The most notable example is "Tall Tales": Sam and Dean are bickering like an old married couple and are annoying the hell out of each other so Bobby is the only one to see that the Trickster is pranking both of them.
Jackson in that everybody but him cheers on Scott's rather miraculous change of athletic ability. He almost seems frustrated that no one else thinks that something is wrong with Scott - even his girlfriend starts hitting on Scott.
However, as the story progresses, Jackson seems to be falling very far and very quickly on the scale of actual sanity.
Now the role has been doubly passed onto Stiles (for Jackson and Scott), and Lydia (for Allison).
Although I wouldn't really call Lydia sane as of the second season.
Of Derek's pack, Boyd is easily the most level-headed of them, including Derek.
The Thin Blue Line: Although Fowler usually has his head screwed on straight enough, Habib's a lot more down-to-earth and self-aware.
WKRP in Cincinnati: Andy Travis. Created as the show's protagonist — that's him being referred to in the theme song — but the writers soon realized he was too dull to drive many plots and made it into an ensemble series with Andy at the bemused centre of the crazy.
Young Dracula: Chloe would often be the only character to notice how bizarre the situation actually was.