Soul Eater: Maka. She's the only one of the three main characters who's practical, mature and focused. Black Star is incredibly arrogant, self centered, obnoxious and loud, while Kid is an obsessive freak about everything being symmetrical.
Some would say that Seras Victoria is one of these as well.
Mazinger Z: Professor Yumi, Shiro and sometimes Boss alternated in filling the role. The first one was the director of a research lab where Mad Scientists (the funny version) and Hotblooded, hardheaded teenagers ran around. The second one was KoujiKabuto little brother, and often showed more brains than his rash, loudmouthed, tactless older brother. And Boss... well, even though he was Plucky Comic Relief, often he was the one had to talk Kouji or Sayaka into reason.
A serious example happened in episode 7: while Sayaka was going through a Heroic BSOD and losing her will to fight, and Kouji was playing Achilles in His Tent, Boss was trying to remember them over and again there WAS one Mechanical Beast coming, and all of them would die if they didn't fight back! Later Kouji and Sayaka were apologizing at each other and Shiro noted boss was the only was right in that situation.
Azumanga Daioh. Yomi is probably the most sane person in her group, followed by Chiyo.
One Piece. When the antics get too crazy, Nami or Zoro will yell at the crew about the absurdity of whatever is happening.
Robin to a somewhat lighter degree. She usually just smiles and admires their free spirits, but she once outright refused to go with a stupid battle formation because it looked too silly, much to the crew's surprise.
Vivi, when she was still a part of the crew, was often one of the voices of reason alongside Nami and Zoro.
One Piece somewhat subverts this trope, as there are plenty of Only Sane Man situations, but the actual crew member playing the sane man varies wildly - depending on the situation it could be any of them. Even Luffy had one once (Chapter 166 or Episode 104).
Less humorously, this is what Coby, Smoker, and Tashigi are when they realize that the marines have pretty much lost it and are going on a petty massacre of pirates when they could be saving lives. At the time, their initial goal of killing Ace was already completed, and Whitebeard was dead as well, plus there was Blackbeard who was going on a straight out rampage and was threatening to destroy Marineford. Instead of helping with that, they (along with a bunch of high-ranking officers such as Akainu) instead start killing pirates who don't want to fight anymore (or have any reason to) all in the name of justice. Coby calls them out on it, which nearly gets him killed.
Trafalgar Law automatically became this once he allied with the Straw Hats. While it may seem that Law could manipulate the crew easily, especially given the massive Chessmaster credentials he got during the Punk Hazard arc, the truth of the matter is he was never in control of the alliance to begin with. Any attempts to make them his Unwitting Pawns were pretty much nil, seeing as he did not account for their antics or Luffy's "I do whatever the hell I want" attitude. That's the downside of being the normal (or at least closest to normal) side of the alliance.
In How to Read 13, (the Death Note encyclopedia), after the quiz, it shows that Soichiro Yagami is the only one with common sense.
Re-l from Ergo Proxy to the point of being The Comically Serious. This is, however, mostly due to her lack of social interaction with regular people, and she eventually comes to realise that everybody has flaws, and sometimes it's best to just accept it and go with the flow.
Haré often takes this role in Haré+ Guu. Sometimes his desire for normalcy makes him abnormal.
Hen Zemi: Nanako is often the butt of everyone's odd fetishes, although she does have her own, hers are just overshadowed by the others.
Ikuto of Nagasarete Airantou is convinced (for a while, at least) that he is the Only Sane Man on the Airantou. He is certainly the only man. After a while, he decides that there's little point in taking issue with all the crazy, and just rolls with it. Still doesn't believe in ghosts (despite meeting one or two) and insists on calling the local kappa a turtle.
In Bobobo-bo Bo-bobo, Beauty (and occasionally Gasser) seems to be the only one capable of noticing how strange everyone else is acting, despite a few rare occurrences where she acts strange as well. Softon applies too, being slightly more serious than everyone else, but when your head is shaped like poop/chocolate ice cream, it's hard to be taken seriously.
Princess Tutu has a mild version of this trope: Ahiru, as a duck who used to live outside of the story, is the only one at first to notice the odd things that happen in town, like a cat being a ballet instructor and an anteater being one of the students. She quickly gets used to the oddities of the town, however. Autor might also qualify - he's the only character to realize that he's living inside of a fairytale that isn't a character in the story.
Chisame from Mahou Sensei Negima!. At the beginning of the series, she is apparently the only one to notice, or care, that the students of Class 3A are... a little on the odd side. Not odd as in 'a little eccentric', odd as in 'that girl is obviously a robot!'. On the bright side, she noticed that Negi seemed to have an unusual respect for her, which proved useful when hegoesberserk and she has to snap him out of it.
In Excel♥Saga, Watanabe was at first the only one who complained about working for the Department of City Security, the absurd situations they were put in, how ridiculous their equipment was, while everyone else kept quiet because his co-workers were either idiots or were just okay with getting work. Then things got more dangerous and Watanabe went insane, and for a while Sumiyoshi was the only one upset at how unstable everything had gotten. He calmed down once Matsuya started working to keep everyone out of danger.
Shinpachi on Gintama, even though he sometimes get carried away by his friends' weirdness.
Tsuna from Katekyo Hitman Reborn! - though as the series goes on, less so, since the insanity is apparently contagious.
Azusa in K-On appears to be this at first, and rails on the other members of the band for not being what she expected (in the respect that they rarely practiced, and sat around eating cake and drinking tea), but eventually accepts that their own quirks are what makes them what they are, and later on gets shocked when it gets pointed out to her how she became just as quirky as them through association.
Nodoka is the real only sane woman. She's the only one that does not fall to the other girls antics.
There was also Mio Akiyama, who keeps the Light Music Club in check. Unfortunately she is overpowered by Cloudcuckoolanders and Genki Girls. This was until Azusa Nakano came in and shared that position.
Yukiteru of Mirai Nikkiwas the Only Sane Man until his parents died and he was convinced he could make things right by becoming God; "sane" being defined as "not a homicidal maniac". He may have a Freudian Excuse, but when you massacre orphans in the name of becoming God, you lose your "sane" title.
Very ironically, the end reveals that Yuno was the only "true" sane person all along (yes you read that right), especially when the various reasons for her actions (her inability to save the First World Yukiteru and the fact everyone in the Second World was as good as dead anyway) are brought to light. Indeed, Mirai Nikki is one of the rare cases where it's not the ax wieldingYandere that's nuts, but the world around her with its warped metaphysical laws and facade of "normal" society.
Sanpeita from Kemeko DX is apparently the only sane man left on the planet. While his classmates have a problem with their short, Gonk-ish, supernaturally powerful and totally insane teacher insisting she's married to him, their problem is generally jealousy. His childhood friend, who is aware that the teacher is a cyborg suit containing a hot girl, tends to ignore this in favour of protesting to the impropriety of the relationship. His sister is in raptures over her cool "onee-sama". His mother is totally oblivious. Meanwhile Sanpeita cries about how his normal life is being totally destroyed...
Shuichi, Kirie's boyfriend in Junji Ito's horror manga Uzumaki, is often the only person who sees the spiral phenomena happening all over town as bizarre, and who realizes the danger the town's in. He sometimes slips into spouting Cassandra Truths in a "You Have to Believe Me!" voice, though by the manga's start, he's largely given up on trying to convince the public and focuses instead on just protecting Kirie.
Although Maeda seems to get this kind of treatment somewhat too, seeing how he is the only one of the main cast who seems to care about stuff like that you need nine players to play baseball, AND that gorillas and motorcycles don't count as players, something Kamiyama seems to completely ignore at that point.
The cameraman in Millennium Actress is the only person to notice that they are acting out scenes from old movies.
Kagami from Lucky Star is probably the most sane person in that entire universe.
Lucy is pretty much the only sane person in the guild in Fairy Tail. When she points out what's wrong people act like she's a loony. Later she is joined by Max, Warren, Charle and Pantherlily.
Takamine Kiyomaro from GashBell often takes this role, especially when Kanchome, Folgore, or Riddle professor are around. One particularly hilarious occasion was in his dream after which he loses Answer Talker.
Taizo in The Legend of Koizumi is the only one to realize the patent absurdity of a world where politics are decided by mahjong, they really can rebuild him, and the Nazi high command not only survived WWII, but have been living on a moon base this entire time.
Hito Nami from Sayonara, Zetsubou-Sensei, due to her excessive normalness. Harumi and Abiru are close behind, though still fall prey to their eccentricities (former is a Yaoi Fangirl with an ear fetish and the latter is a Bandage Babe due to a severe tail fetish).
Higurashi no Naku Koro ni features both subverted and straight examples. Keiichi is implied heavily to be one early one, then subverted twice with what happens in Tatarigoroshi-hen and when it's revealed what was really happening in Onikakushi-hen. Rika, however, is an excellent example as she's immune to Hinamizawa Syndrome, the Hate Plague that has been driving everyone crazy and turning them into murderers in the previous arcs. Mion should count, since we've never seen her snap and go crazy, however, she's a more complex example as Rika has alluded to worlds when she did; since she's been traveling worlds for hundreds of years, she can be trusted on this one.
Actually, Rika says in the TIPS that Mion never snapped. Rika has snapped in one or two offscreen kaekras, but was killed before she harmed anyone.
Bleach has surprising few examples that truly fit this trope, despite all the crazy situations that happen throughout the course of the story.
In Episode 303, the shinigami play a New Year's karuta card game. It's originally supposed to be based on a real game but Yachiru doesn't bother explaining the rules. Players get ejected on the basis of made-up rules and things quickly degenerate into chaos with kidou and shikai being liberally thrown around. And then bankai reduces everything to rubble. Hitsugaya is the only sane man.
In Episode 335, Kira Got Volunteered by Hisagi to host an New Year's divisional kite-flying bonding exercise, but Hisagi sabotages it to spice things up. Things escalate from gentle bonding to all-out war. Only sane man Kira attempts to end it but accidentally destroys Yamamoto's barracks. Yamamoto's Unstoppable Rage ends the chaos with the effectiveness of a Fantastic Nuke.
Juu from Denpa Teki na Kanojo is this in a more literal sense: he's one of the few characters who can claim a clean bill of mental health.
Hatori from Fruits Basket seems to be the only sane one in his group of friends (the other two are Shigure and Ayame) Whenever they start their antics, he chimes in with a line like "How could anyone sleep with all this noise?" or "Don't the two of you ever stop?"
Kou of Arakawa Under the Bridge perceives himself to be this, and has decided it is his job to restore some measure of sanity to the other people living under the bridge. However, his sanity is largely relative to the residents living under the bridge...
Jim Hawking from Outlaw Star is frequently the only one to see the insanity in Gene's incessant Indy Ploy's.
He's a cold-blooded Sociopathic Soldier, but when compared to his lunatic partners, Auel, and Stella, Sting Oakley of Gundam Seed Destiny is a pillar of stability and reasonableness. He's the only one to Know When to Fold 'Em, and he typically acts as the brains of the outfit, keeping the other two from going totally over the edge. When they die, he finally snaps.
Sting owes something to Gates Capa of Zeta Gundam, the only remotely sane Cyber-Newtype, whose job seems to have been keeping Rosamia stable. Zeta also features Emma Sheen and Henken Bekkener whose Ensemble Darkhorse status is owed almost entirely to their being a pair of reasonable, rational, competent adults in the childish Dysfunction Junction that is the AEUG.
Koushi Inuzuka of Sumomomo Momomo is the only one aware that there are laws against the types of shenanigans that regularly break out, and he regularly quotes them for good measure. He also initially objects to the impossibility of some actions, but eventually just settles into being a Deadpan Snarker about it.
Hyuga Koushiro is more or less this in Kamisama Dolls, especially among the Seki. Utao and Kirio are just children, Aki's a homicidal maniac and Mahiru is... not all together there. Kyouhei tries to be this but still has lingering, unresolved issues from the same event that broke Mahiru and contributed to Aki's issues.
Konan from Naruto is arguably this when it comes to the Akatsuki.
She's the sanest individually. If we are going by pairs, arguably the most normal (and by default, sanest) is Itachi (who would have also been an individual Only Sane Man if it weren't for his fucked up plan involving Sasuke) and Kisame (who doesn't count individually because he's extremely bloodthirsty). This is because Konan is loyal to and the partner of a guy with a massive God complex, and the other pairs include a pair of immortals who argue all the time, with one being greedy as hell and the other a sadomasochist with an extremely foul mouth, and two artists who argue over philosophies over art (one who likes to blow stuff up and the other likes to make human puppets). Then there's the last pair, which includes a plant guy with a likely schizophrenia disorder, and the Plucky Comic Reliefwho's really the mastermind of the entire organization, and took part in an Assimilation Plot that sounds like it was made when the person was both high and drunk just so he could see his dead crush.
There is at least one sane man among every Dex Holder group in Pokémon Special:
Stoic trainer Green compared to nice guy Red, flirty ex-thief Blue, and shy girl Yellow.
Aloof Silver or strict Crystal compared to hotblooded troublemaker Gold.
Bratty midget Emerald can't stand prissy Ruby's and wild Sapphire's endless bickering.
Impatient straight man Pearl, spoiled heiress Platinum, and cheerful glutton Diamond have all had a chance to be the normal one among the trio during their journey.
Snarky Cheren or responsible White compared to ditzy Bianca and loudmouthed dreamer Black.
The Budokai announcer and dog king of the world in Dragon Ball are probably the only persons in the world not connected to the main cast to realize that Mr. Satan's claims of beating Cell are nonsensical and that it was much more likely he was defeated by one of those super powerful people who had fought in the previous Budokais and defeated Demon King Piccolo.
In Corseltel no Ryuujitsushi and sequels, Nata the darkness dragon is this to the other baby dragons. In terms of older dragons and mages, he's still just a baby, though.
While multiple characters in Rave Master posses common sense, the vast majority quickly turn out to be Not so Above It All and will join the wackiness as soon as someone more serious than them temporarily joins the team. The only person who can make everyone else take on a comic relief role is Sieg Hart, who's somewhat exasperated when he realizes that the two most important people needed to stop Demon Card are also the two flakiest members of the cast.
Though not intended to give this effect, Prince Odysseus in Code Geass appears to be the only sane man in all of Britannia. Though frequently called "mediocre" and depicted as indecisive and having a flat, unexcitable temperament, these aren't necessarily bad things in governance, he's only indecisive insofar as that he is basically the only character who is open to more than one possibility while dealing with complex, volatile geopolitical situations (nobody in this world ever thinks to abandon or even amend their absurdly convoluted plans when everything falls apart, despite millions of lives and entire economies often being at stake contingencies plans are completely unheard of) and reacts to things in a calm, reasonable manner rather than standing up, screaming and launching into a silly hand movement. He acts like a normal politician in world-power that's seemingly run entirely by supervillains. He's always the voice of reason, he's never paralysed with shock and never raises his voice. He may very well be the only reason the Holy Empire of Britannia hasn't destroyed the world in a barrage of explosive shells, off-model faces and ridiculous poses.
Homura from Sekirei is often stuck as this, among Minato's Battle Harem. One of the other rare male Sekirei, Mutsu, seems to be cursed to be this wherever he goes. When the two meet briefly, the later realizes the former lives at Izumo Inn and as such is dealing with hisoldcomrades. He expresses his sympathy and wishes the other good luck in dealing with "those witches".
Ami in Toradora! is the Only Sane Girl. Sure, she lacks tact, but if you actually listen, you realize what she says makes a lot of sense. Plus, considering her company is Minori and Taiga, it's hard not to look sane by comparison.
In Nerima Daikon Brothers Hideki is the only one who reacts to the craziness around him the way a normal person would. Subverted with Ichiro, who appears calm and composed but whose behavior is often downright weird.