Only Sane Man/Western Animation

The "Chicken Boo" sketches on Animaniacs had only one person realizing that Chicken Boo was actually a giant chicken in a ludicrously flimsy disguise. Nobody would believe them, and even when Boo's deception was eventually revealed (by some event unrelated to their efforts), they never got any credit for their insight.
- Animaniacs also had Scratchensniff.
In Invader Zim, Dib is the only one who knows or cares that Zim is an alien, no matter how obvious it becomes. Gaz is a twist; she knows perfectly well that Zim is a would-be alien conqueror, and doesn't care because (in her own words) "he's so bad at it."
- Arguably Gaz is even more so than Dib. While Dib obsesses over Zim's constant plots, Gaz is all too aware that Failure Is the Only Option for the Chaotic Stupid Zim and is content with just letting him screw himself over.
- Originally, both of the Tallests were shown to be somewhat moronic, but Red's Divergent Character Evolution was sending him in this direction, best evidenced in "Backseat Drivers from Beyond the Stars".
The Simpsons episode "Homer's Enemy" featured the character Frank Grimes, who is the only one of Homer's co-workers who recognizes his pathological boobery. He tries to point it out to everyone, but to no avail. He eventually works himself up over it so much that he has a nervous breakdown, during which he accidentally electrocutes himself. After some fans complained his treatment seemed a little mean-spirited, DVD commentary explained Frank was mainly a device to show a normal person couldn't function in the show's universe.
- Lisa Simpson also often finds herself thrust into the role of only sane...girl. Quite often, when her family (or occasionally, the entire town) is caught up in some insane course of action, she's the one who points out how absurd things have become. Her comments are usually ignored.
  - The straight man role is usually traded between Lisa and Marge. Sometimes even Lisa's as crazy as the rest, and vise versa.
- There are even times that only the Simpson family who really knows what's wrong, and have the guts to do something about it.
In Sealab 2021, Doctor Quentin Q. Quinn is the Only Sane Man. At least in the beginning of each episode. Often the sheer insanity of everyone else becomes contagious.
On The Smurfs Papa Smurf tends to be the Only Sane Smurf, seemingly able to resolve any conflict among the younger Smurfs with simple words. In fact, some of the worst disasters ever to befall their village happened when he wasn't around.
Frylock on Aqua Teen Hunger Force. In relative terms, anyway.
Hank Hill of King of the Hill is a shining example. In his version of The Rashomon, he sees himself as the only adult among children, and in a hallucination caused by varnish fumes, he imagines the rest of the world as helpless rodents on a Whack-a-Mole board.
Bob the Tomato, in the Veggie Tales cartoons -- in the opening and closing sequences where he plays himself, he consistently provides a voice of reason on the show. During the actual episodes, he is almost invariably cast in the role of a character who is competent and sensible, unlike most of the others.
Louise in Slacker Cats is the voice of reason throughout the show but Buckley does zig-zag between being this and one of the many crazy people/cats throughout the show.
Maurice in The Penguins of Madagascar. Really, just he and maybe the chimps are the only ones.
- And occasionally Private. For example: "The Red Squirrel." And Marlene, all the time (except when she's outside the zoo grounds or talking about Spanish guitars). Rico once, actually. In "The Hidden" he keeps Kowaslki and Skipper focused on the task at hand (finding their friends). Skipper, never. Kowalski, almost never. Though he thinks he is. Julien, no way. Mort, uhh... No... Roy comes across as sane, however.
Number 5 of Codename: Kids Next Door:

Number 1: Abby, we both know you're the only one on this team with any real common sense. When I'm screaming that we're being invaded by tiny, kid-eating leeches falling from the sky, you're the one telling me it's only snowing.

Rocko from Rocko's Modern Life seemed to start out this way, a hapless bystander trying to use logic to navigate a thoroughly insane environment and oftentimes failing. Strangely, he seemed to lose this as the series also took a turn away from being as surreal as it once was.
Stan from South Park. Kyle has the appearance of being sane as well, but unlike Stan, is unable to understand just how moronic the actions of all the insane people around are. Among the adults is/was Chef.
- In the "Pandemic" episodes, Craig took on this role, as he pointed out the ridiculous situations the characters were always getting into, delivered entirely in deadpan style. Every character except for Stan has derailed into being insane, stupid, or a jerkass. Kyle really doesn't apply for the Only Sane Man because he is just as immoral and sometimes cruel as Cartman when It comes down to it. Whenever Cartman is involved, Kyle is far from sane to say the least.
  - Kyle's tendency to have quite a temper to the extent of being damn near bipolar excludes him from this on a regular basis, but there are instances where he plays this straight. The episode concerning the economy and the absurdity of aestheticism is a crowning example. Even "You're Getting Old" has him playing this to Stan of all people.
Shnitzel from Chowder is usually a good example, though he does a have a few lapses into the weird goings-on in the show.
On The Fairly OddParents, Wanda is usually the only one aware of how insane everything is, or knowing of how bad everything is going to end up. (Just keep her away from chocolate.)
- This fact is often pointed out by Timmy and Cosmo, but they still continue with their antics despite that.
On the Animated Adaptation of Wayside School, Todd is repainted as the only one using real world logic while everyone else is using Wayside's logic.
Mac in Foster's Home for Imaginary Friends. Even when Frankie gets into the house's general wackiness, Mac doesn't without getting into Mr. Herriman's extreme. However, being able to manage this at the age of 8 doesn't come without its costs...
- You mean like sugar?
Dukey from Johnny Test. He's the only one aware of the bad side effects of Susan and Mary's experiments (besides Hugh) and he knows when a fun time can get dangerous.
Although only sane person is stretching it, Foxxy Brown on Drawn Together was once accurately described as "The only one here who's not completely retarded."
Sokka in Avatar: The Last Airbender more than once, especially in The Cave of Two Lovers and the Painted Lady, and though the latter was combined with Kick the Dog, he was right...
- Even more so Suki, when she is with the group. She's the only of the main and recurring characters who could be considered to be normal. She takes it quite gracefully, though.
- Also Katara sometimes, most notably in the episode The Runaway, in which she is the only one who realizes that excessive scamming of scammers will come back to bite them eventually. Toph started the whole thing, and Sokka being involved is not horribly surprising, but Aang should have known better.
The Chief and sometimes Stinky Diver from Action League NOW!
The Venture Brothers has Doctor Venture, who often tries to be the only sane man, but his childhood always catches up to him. Usually, the roles ends up being filled by one-shot bad guys that question the Venture Team's logic. You know you have problems when a flamboyant super-villain is judging you.
- Brock fills this role quite often as well, in spite of murdering multiple people on an almost daily basis. He's generally the sanest, most level-headed and most competant member of Team Venture, so it's not hard for him to slide into the role and see just how absurd his life and the lives of those around him are.

Dr. Venture: (As a party being thrown by his brother sits on the brink of collapsing into madness) Huh. When you're not the one in the middle of it all, it's actually pretty obvious.
Brock: Welcome to my world.

Family Guy: Brian Griffin is often portrayed as the Only Sane Man, although everyone who isn't Peter ends up playing the Only Sane Man once or twice.
Kim Possible has Shego. Any episode featuring Doctor Drakken will have her standing off to the side making running commentary on the absurdity of his plan, or how it will inevitably fail because of one major overlooked factor. It is also implied in the episode 'Go Team Go' that this was the reason for her leaving the team of superhero siblings and going bad.
In SpongeBob SquarePants, Squidward Tentacles and Sandy Cheeks are more or less the only creatures in Bikini Bottom with sense. Spongebob also tends to be this whenever Patrick Star is the stupid one.
- Plankton often shifts into this when dealing with Spongebob.
On Jimmy Two-Shoes, the titular character is the only one that's both halfway compatent and doesn't want to beat down anyone who looks at them funny.
Gwen from Total Drama is one of the most normal contestants, and qualifies as this for most of Total Drama Island. Noah is more of a classic example, especially when you compare the rest of his team in World Tour (excluding Alejandro, who gets this whenever Noah's having fun). As of World Tour, Noah is the best example.
As quoted above, Sally seems to be the only denizen of Halloweentown who even approaches the realization that people don't want to be scared or attacked on Christmas.
Despicable Me and Megamind would have two in each movie: Dr. Nefario and Margo in the former, and Minion and Roxanne in the latter.
Captain Star's First Officer Scarlett is the one who actually gets the work done between an egotistical captain, a fish obsessed cook and an engineer whose isn't that crazy, in fact, he's perfectly sane. He has nine sane heads on a single body.
Shep from Horseland compared to most of the cast.
In the X-Men animated series, the largest of the sentinels (robots designed to capture people) is called Master Mold. He/It starts trying to take over the world. When reminded by his creator that his mission was to protect humans from mutants, Master Mold says, "That is not logical. Mutants are humans. Therefore, humans must be protected from themselves." Not a bad realization for a robot antagonist that Professor X called "the embodiment of human hatred" that was created by another antagonist.
Danger Mouse. He has to deal with a Cowardly Sidekick (Penfold) and an addled, near-senile superior (Colonel K). DM will either take the initiative and stop their banter or just throw up his arms in defeat.
In CatDog, Mervis and Dunglap are the only normal characters in the series.
Panda of Skunk Fu! knows what's best for Skunk (seeing that he's his master).
Li-Chun of Hero: 108 is alway calmer than the rest of the cast.
Bob of Bob's Burgers is the family's only denizen member.
Twilight Sparkle in My Little Pony Friendship Is Magic is generally the most level-headed of her friends, but later episodes demonstrate that even she's not immune to lapses in judgment.
- Spike takes over whenever Twilight takes a holiday from the role.
- In the play put on in Hearth's warming Eve, Twilight, Fluttershy and Applejack all play this too their leaders. Especially Applejack's character Smart Cookie, who has to deal with a political leader who's completley stark raving mad.

Chancellor Puddinghead: I mean, how would we survive if I just suddenly shut up?
Smart Cookie: Heaven forbid that should happen, your Chancellorship.

- in g3 my little pony Kimono is your pony.
Titan Maximum deconstructs the idea of the Only Sane Man with Gibbs. Being the only competent member of the former Titan Maximum team eventually drove him into hating everyone else on the team and performing a Face Heel Turn on the first episode. The fact that he's the Only Sane Man makes him a very effective villain, albeit an incredible Smug Snake.
Charles Offdensen from Metalocalypse. We actually see him go through the three stages in the background; by the time season two ends, he's learned to roll with it. Season three sees him realize that he's not the Only Sane Man. He's just the Only Sane Man on his side of the conflict.
There are several candidates from Phineas and Ferb, but, at some point, most characters get swept up in the show's general inanity. Despite being, well, a platypus, Perry plays the straight man to Doofenshmirtz and Jeremy usually embraces the show's zaniness, but every now and then the watcher can see the Flat What on his face. For example:

Phineas: Great! So if we just find mom, we'll find both Candaces!
Jeremy: Uh... tell them I said hi...?

Transformers Prime actually has this in one of its human characters, Jack Darby. Unlike almost every other human in the Transformers universe, he knows that getting yourself involved in the front lines of an alien robot war is more dangerous than awesome. In fact, he tried to avoid all contact with the Autobots at first before some convincing.
- The villains have Soundwave, who doesn't share the sociopathy or megalomania of most of the other Decepticons and is far and away Megatron's most loyal and competent henchman. Be warned: This trope does not in any way make him less dangerous than the others. At all.
Mickey and Minnie are this in House of Mouse.
Karl Pilkington of The Ricky Gervais Show. Whil Ricky and Steven laugh about his thoughts, he just didn't laugh
Chris from Dan Vs unless food is involved. "I'd kill for [bacon]" isn't something most sane people say after all.
Edd (also known as Double D) from Ed, Edd 'n Eddy usually fills this role.
Hermes often takes this role in Futurama, most notably in Bender's Big Score where he's the only person to realise the crew is being scammed. He's also the most consistent critic of the incompetent Zoidberg. Depending on the episode, though, Leela, Amy, Bender and even Fry have occasionally filled the role.
Marlin in Finding Dory (2016 sequel to Finding Nemo). Marlin is contrasted with Idiot Hero Dory and his son Nemo, who is now apparently somehow Closer to Earth. At the end of the movie, Marlin learns a Valuable Lesson about how to "follow his heart" instead of using reason.