Black Butler goes out of its way to prove this of Ciel. To the point where Joker in the manga gives him the stage name of Smile in an ironic sense...but then finds out that Ciel is a damn knockout. It seems as though Elizabeth's entire point of existing is to make him smile again...and she is one of the few people who can get a genuine smile.
Haruhi herself starts out the series with a perpetual frown (hence the title of the first book, The Melancholy of Haruhi Suzumiya). Kyon describes her first smile as "reminiscent of a blazing sun in an equatorial sky," and later uses adjectives like "divine" and "cherry blossoms in full bloom" when referring to it.
Yuki Nagato, resident Emotionless Girl, gets one. This◊ picture from the Disappearance is a plot point in said movie and the novel it's adapted from — seeing Nagato smile is what leads Kyon to reflect on whether recreating the old world would be the right thing to do. Yuki/Kyon shippers tend to emphasize this point.
Subverted with Kuyou (resident Stringy-Haired Ghost Girl, in novel ten, during the fight with Ryouko Asakura. While Kyon comments on how smiling makes her whole appearance different, even beautiful, it still comes off as profoundly inhuman.
And being another Rei Ayanami Expy, in Rurouni Kenshin, every frame which features Tomoe is gloom, grave, whose inability to smile caused her original fiancee to sign up as a bodyguard for a powerful samurai and get himself killed by Kenshin, can be seen smiling twice: once, when she dies, and once in a vision asking him to continue living life to the fullest.
Enishi, the villain of the same arc, is willing to kill for that smile.
Izumi Himoru from Princess Nine pulls off this trope at the end of the final episode, where she hugs Ryo Hayakawa after the team lost in the in the quarter-finals, to cheer her up, with the hope of winning next year.
Tatsuki from Hands Off smiles a grand total of one time in the whole eight volume manga.
Borderline example: In Full Metal Panic, Sousuke is not unattractive even in Perpetual Frowner mode - it's even directly addressed in the novels, where he's still considered very attractive, but very unapproachable, as his eyebrows are in a constant state of slanting down angrily, and his lips are always pressed firmly in a grim, straight thin line. But during the moments where his face relaxes and he lightens up, people who see him like that tend to have their heart racing BIG TIME.
Like Sousuke, Darker than Black's Hei generally racks up a Chick Magnet count even without a real smile (he has plenty of fake ones, but they're just that — fake), but if Amber's testimony is anything to go by, a genuine one can make sociopaths fall for him.
Amber: There was one time that he actually laughed from the bottom of his heart. His smile captivated me. That's when I thought to myself, "You really did it this time." I felt as if I would do anything for him.
In Battle Royale, Hirono, the second in command bad girl in Mitsuko's gang is depicted as this. Despite having a punk hairstyle and being very nasty normally, when she meets up with protagonist Shuya (whom it's implied and revealed that she actually has a crush on), she ends up showing a smile from her heart. This leads to Shuya realizing that she's not all bad, and throughout the series, one of his greater regrets is being unable to save her. It pretty much directly follows this trope when he describes her in this page.
Sawako from Kimi ni Todoke usually looks damn creepy what with resembling Sadako and all, but her genuine warm smile was one of the first reason Kazehaya fell in love with her. When she tries to force herself to smile though, it, ah...doesn't work.
Akane Tendo in Ranma ½. Even Ranma freely admits that she's cute when she smiles.
In the Pokémon anime, Marley succeeds big time at doing this when a Shaymin she saved before crawls into her sleeping bag.
In all of Ghost in the Shell, the Major smiles only once in Stand Alone Complex, when Batou challenges her to a friendly wrestling match. Her reaction is only to smile at him◊ and then using her brain implants to wirelessly hack into his cybernetic arm and bash his own fist into his face. Otherwise, she has a general rotation goes between 'Confusion', 'Detachment', 'Disgust', and 'Frustration', with occasional visits to a subtle smirk of victory. Even her obvious rage face only makes an appearance in one episode. But this trope still applies, because the few times she actually shows happiness it is absolutely heartwarming, and it's quite possible this is why Batou hesitates. Alternately, it could be that he was simply very confused as to why she was smiling. Averted in the manga where she's quite expressive.
Umi no Misaki: Nagi feels this way about all three of the girls. Karin is seemingly perpetually happy, which Nagi finds one of her most endearing traits; He likes Shizuku when she is able to smile and relax; It's a positively heartwarming moment for him when he gets Soyogi to open up and smile.
In The Prince of Tennis, there are exactly two (perhaps three) instances in which Tezuka Kunimitsu can be seen smiling: one is after a very symbolic match with his best friend Fuji Shusuke, the other is after his team has just won the National Tournament against the supposedly-unbeatable Rikkaidai. The last one, Inui Sadaharu even felt necessary to get on tape.
In Vampire Hunter D, D's (very, very rare) smiles are apparently worthy of historical documentation.
If that boy could have seen it, he would have told people for the rest of his days how he'd been the one to bring it out. It was just such a smile.
Konzen, from Saiyuki, like his later incarnation, Sanzo, is very pretty but has a permanent scowl. Goku, his surrogate son, thinks he'd be beautiful if he did smile. Subverted, though, as when he finally does Goku decides it doesn't suit him at all, even though it is a completely genuine loving smile. Doubles as a crowning moment of heartwarming, as essentially Goku is saying he loves Konzen as he is.
Mika and Koharu of Koharu No Hibi just want Akira to always be happy and smile. And Koharu is normally a creepy Stepford SmilerYandere but she does genuinely smile here without any insanity or creepiness behind it.
Invoked in Sensual Phrase. Aine is a Cute Bookworm who doesn't gives smiles a lot due to shyness, but when she does, she's very cute... But when she's chosen as a model specifically due to this, she cannot smile as cutely as before, which causes her quite the angst.
Discussed by Guts and played for drama in Berserk. After all of the crap that they had gone through since the eclipse, all that Guts wants to do is to make his insane lover Casca smile (though she is attractive regardless of if she smiles or not). But after he nearly strangles and later almost rapes her on account of being influenced by his Enemy Within she is now fearful of him and won't even let Guts touch her with tenderness anymore. It's more heartbreaking because seeing her smile is the only thing that makes Guts himself smile nowadays, and it kills him to see how Casca fears and detests him now.
To add to the above, in volume 28, Guts briefly gives a true smile (bonus that his eyes are shining and not dull like they usually are) upon hearing from the Skull Knight that Casca might be cured of her insanity once and for all if he gets her to Elfhelm. Schierke, who is present, gives a soft reaction of happiness that she was able to see Guts smile like this.
Applied several times with Chairman Morinomiya in Gakuen Babysitters. Always acting like a grouch, the few times she smiles reveals quite a lot.
Sen Yarizui from Ben-To is usually The Stoic, as well as a Rei Ayanami Expy (right down to the pale blue hair and red eyes), but her features soften considerably when she offers up a sincere, heartfelt smile.
The Ironic Hell for suicides in Wristcutters is just like the real world, except that nothing works, everything's kind of gray, and it's impossible to smile. After seeing nothing but despondent looks from all of the characters for the entire movie, the last shot—the leads smiling at one another—is like a wash of beautiful sunlight.
Aurelia in Love Actually. In the commentary, Richard Curtis says he immediately decided to cast Lucia Moniz after she smiled during her audition, and immediately went from dour plain woman to radiant beauty.
Sense And Sensibility. The Colonel, as played by Alan Rickman. He's looked like the incarnation of gloom throughout the film, and then, in the final scene when he's married Marianne, his face lights up and he seems to have dropped about ten years of age.
Also Elinor, as played by Emma Thompson, who lights up with one bright, beaming, radiant smile at the end of the film and immediately negates about five minutes' worth of Inelegant Blubbering. Edward's besotted expression says it all.
Pride and Prejudice. Mr. Darcy as played by Matthew Macfadyen in the 2005 version. Throughout the entire film, he's extremely solemn, reserved, and aloof, then near the end when he's about to ask Mr. Bennet if he can marry Elizabeth, he smiles - ever so slightly - and it lights up his entire face.
It gets even better when you think about it. The last time Darcy pointed something electronic at Thor she had him with a car and tased him and now she's doing it again, but this is Thor we're talking about. People from Asgard hit each other with cars as a form of greeting. Him just smiling like that is even more hilarious because this is just another day.
And, surprisingly, Loki, albeit in a deleted scene. Even if you doubt his motives for going with Thor, it's one of very few scenes in the film where he looks genuinely happy about something (if only for a few seconds). And damn does he look adorable.
Subverted early in the Pirates of the Caribbean cycle when the creators had to cut every incidence where Commodore Norrington smiled. (They are still in the Deleted Scenes on the Curse of the Black Pearl DVD.) Jack Davenport's smile was charming enough for the writers to go Draco in Leather Pants on their own character and start re-writing him accordingly- these scenes were cut out because the character wasn't supposed to have so much of the audience's sympathy.
In Robin McKinley and Peter Dickinson's short story collection Water: Tales of the Elemental Spirits, the main character in one the stories is described as one of these ("Her face lit up with kindness and humor and intelligence.") The same phrase, in an non-ironic echo, is used to describe her Love Interest.
Meghan Whalen Turner's short story collection Instead Of Three Wishes has the protagonist of the titular story fitting the trope. Her honest smile "could make flowers bloom early" and stuns the elf king who's known her for a while when he finally sees her smile for real.
Turner likes this trope. Helen/Eddis from her Queen's Thief series is described as mannish and decidedly NOT beautiful... but even the dour Magus admits that she has a lovely smile.
Number Ten Ox's description of Lotus Cloud in Bridge of Birds. He goes on about how she looks like just a normal, unremarkable peasant girl. "and then she smiled..." and Ox falls instantly in love, as does every other male with a working libido. she is a goddess in disguse, after all.
In the novelizations of Red Dwarf, Kochanski's "Pinball Smile" is mentioned a couple of times; when she smiles she lights up like a pinball machine when you hit the jackpot.
Tuon has also been described this way when she isn't being The Stoic.
The series also provides a male example: when Min first meets the noble who wills become the future King of Tear, she more or less decides that he's ok looking, but with too big a nose. Then he smiles, and she decides that any length of nose can be forgiven with a smile like that.
When Anne of Green Gables was growing up the issue of her beauty was a topic of discussion by many, with the final consensus usually being that she didn't have conventional beauty but did have some unknown element about her that could occasionally blossom out:
"How perfectly lovely!" exclaimed Anne, her gray eyes lighting up until they looked like evening stars, causing Mrs. Lynde to wonder anew if she would ever get it settled to her satisfaction whether Anne Shirley were really a pretty girl or not.
L.M. Montgomery also makes Emily Byrd Starr's smile beautiful yet a little strange at the same time: it completely changes her face, but it blooms slowly. She inherited it from her mother, and it instantly wins over her Aunt Laura, who loved Emily's mother dearly.
In Jean Auel's The Valley of Horses, Thonolan (and everyone else) thinks Jetamio is not-unattractive, until "Oh woman, you are beautiful when you smile."
In The Great Gatsby, Gatsby's smile is described as disarming to a suspicious extent, so transforming that it must be hiding something.
He smiled understandingly — much more than understandingly. It was one of those rare smiles with a quality of eternal reassurance in it, that you may come across four or five times in life. It faced — or seemed to face — the whole external world for an instant, and then concentrated on you with an irresistible prejudice in your favor. It understood you just so far as you wanted to be understood, believed in you as you would like to believe in yourself and assured you that it had precisely the impression of you that, at your best, you hoped to convey.
In Mercedes Lackey's Heralds of Valdemar series this describes the Queen's Own Herald quite well. While Talia is preparing for her wedding, the groom walks in, prompting the following observation from Jeri: "Two hours I spend on her, and in two eyeblinks he makes everything I did look insignificant."
In Gail Carson Levine's short chapter book "For Biddle's Sake," Parsley has such a beautiful smile that not only does it melt the heart of anyone who sees it, but they completely ignore her slightly green teeth (from all the parsley she eats).
Mordion from Diana Wynne Jones' Hexwood is fairly alarming looking - he's tall, gawky, much too skinny, and his face is frequently said to resemble a skull. However, he has such a transformatively warm and beautiful smile that the female protagonist almost falls out of a tree the first time she sees it. Interestingly, at points he is actually seen to deploy the power of his smile strategically.
Occasionally lists several major flaws in Ygritte's appearance and then comments that when she smiles they are much less noticeable.
Subverted with Brienne of Tarth; when she wins a spot on Renly's Rainbow Guard, her delighted smile lights up her face, but it doesn't make her any less hideously ugly.
Played with in the case of Tywin Lannister. He has apparently never smiled since his wife died, which is lamented by his brother and other people who knew him when he was a nicer man. His son Tyrion is present on several occasions when Tywin almost smiles, but instead of being a heartwarming moment, the look is described as "terrible", probably because the context suggests it would be a Slasher Smile. The only time a true smile appears on Tywin's face is after he has been killed, and even then the maesters say the smile is not genuine, and is only a tightening of the facial muscles as the body starts to decompose. Tywin's children (unsurprisingly) prefer to imagine that he is at peace.
Another male example: Sheftu from Eloise Mcgraw's Mara, Daughter of the Nile. Another male character mentions that Sheftu is actually so plain as to be almost ugly, but his smile (along with his super-suaveness) prevents any woman from realizing this.
Tolstoy uses this trope in War and Peace describing Natasha and, to some extent, Princess Marya. At first, it isn't the smile but the expression of her eyes, but in the scene of her second meeting with Nicolas Rostov it's played literally.
From the YA novel Jacob Have I Loved: the protagonist (who is young, island-born and Protestant) has just been chatting with a Catholic, mountain-born widower. At first she thinks he doesn't understand her at all...
"But then, oh, my blessed, he smiled. I guess I knew right then I was going to marry him: God, Pope, three motherless children, unspellable surname and all. For when he smiled, he looked like the kind of man that would sing to the oysters."
In Meg Cabot's Airhead series, the lead Emerson who is actually in super-model's Nikki Howard's body describes everybody (boys and girls) as having this reaction to Nikki's smile, as opposed to what happened when she smiled on her own body.
In Donna Jo Na Poli's Bound (a Chinese "Cinderella" retelling), the prince was amused by Xing Xing's impudence. But once she smiled at him, he was surprised (and delighted) and at once, he was certainly interested in getting to know her.
The first time Ron and Harry notice how attractive Hermione has become is when she laughs and shows how her teeth had been fixed.
Actually, that's only how Ron noticed. Harry only figured it out at the Yule Ball.
Also, Sirius Black. He's not in great shape after breaking out of prison, but after he hesitantly invites Harry to live with him and Harry jumps at the prospect, he smiles for probably the first time since his imprisonment, and it makes him look ten year younger.
In Lucifers Hammer, the character of Eileen is the epitome of this trope: plain-looking, but with a smile that literally stops traffic.
Will Grayson #1: [Jane's] whole face changes when she smiles—this eyebrow-lifting, perfect-teeth-showing, eye-crinkling smile I've either never seen or never noticed.
Prince Henry from The Wild Hunt Trilogy. "Henry laughed aloud. The plain features lit up, became attractively mischievous..."
Margaret in Sisters, No Way. Aishling at one point notices how old she is looking lately but then Margaret smiles and Aishling relaxes. Cindy also remarks "she's not a very attractive woman but she has a nice smile".
In The Dresden Files, minor side-character Vince Graver from Turn Coat was described as The Nondescript until his last scene, when he smiles. Dresden notes that the smile completely changes him from a generic average Joe to someone who could light up the room just by being there.
Note: Yes, it was Dresden describing this, and no, the description in the book had no Ho Yay whatsoever.
In the The Dresden Files, little Inara Raith is described as "pretty" but has a smile that makes her radiant.
Sharpe, who is described as looking perpetually sardonic and mocking due to a Badass Scar, except when he smiles. Which isn't often, and then he looks kind and charming. Since he normally looks like some ancient God of War in mid battle, this is an improvement. It is worth noting that he is always described as roguishly handsome, its just that he looks kind when he smiles.
In Harry Turtledove'sWorldwar, Mordechai Anielewicz finds Bertha Fleishman more attractive when she smiles, normally considering her plain.
But his smile when he turned it on you was quite remarkable. It seemed to be composed of all the worst things that life can do to you, but which, when he briefly reassembled them in that particular order on his face, made you suddenly feel, "Oh. Well that's all right then."
In A Reluctant Queen by Joan Wolf, a retelling of the story of Esther from the Bible, Esther was already deemed a pretty girl but when she smiled, it was enough to warm anyone's hands, as noted by her Uncle Mordecai. And before her interview with the king, she was told by the lady in charge of the girls waiting to see the king to smile because "[her] smile was [her] greatest beauty."
In Tales of Kolmar, the usually serious, inclined-to-brood mage Vilkas is noted by one character to be "pleasant enough, if only he'd smile now and again". Later she says her wish is granted, and she notes "a broad glorious smile that lit up his face and showed the joy that danced behind his eyes". At the end of the series he's more inclined to smiling in general.
Asher in Someone Else's War is a rare male example. His face is heavily scarred, and he spends most of his time shunning emotion as weakness. Smiling (or laughing) makes a huge improvement to his appearances, but he doesn't do it often.
Analyzed at great length when Joan's smile causes Ashe to fall for her in P.G. Wodehouse's Something Fresh.
Arabella Woodhope (later Strange) is described as a modestly pretty girl, but adds that when she smiles, she has been known to outshine known beauties in three counties.
Aragorn in the Lord of the Rings Appendixes has been described to look solemn and grave but when he smiled, he appeared kingly.
His ways were hard and long and he became somewhat grim to look upon, unless he chanced to smile and yet he seemed to Men worthy of honour, as a king that is in exile, when he did not hide his true shape.
Live Action TV
Sarina, one of the Augments in Star Trek: Deep Space Nine that works with Bashir in "Statistical Improbabilities", is catatonic in that appearance due to her senses being too slow for her brain to process. In a later appearance, Bashir is able to get her highly accelerated neural pathways in tune with her senses, breaking her out of her catatonia...and the first time she smiles, Bashir is absolutely smitten with her.
This is a fairly common reaction to Francine's "better moments" in Alias.
The same actress' utterly fake, unconvincing Stepford smile on Malcolm in the Middle, however, is slightly unsettling (her character was a squeaky clean, passive-aggressive control freak for quite a while).
Even cuter, if that's even possible, while wearing her version of an angel costume. Adorable◊
It seems to be a pattern with people named Parker. I quoted Shawn Mullins in regards to Ms. Parker of The Pretender:
"She'd be a whole lot prettier if she smiled once in a while."
In a deleted scene from The Thick of It, we discover that, according to a "worm poll", the public really love Hugh Abbott's smile.
Firefly: Done subtly but very deliberately in with River. River, beingRiver, doesn't tend to smile very often. Thus, the scenes where she is smiling tend to be that much sunnier and happier. It is played especially well in the movie Serenity, where River only smiles once in the entire movie, and that is at the very end while talking with Mal.
A more obvious example is with Kaylee, whose smile makes everything shiny.
Generally inverted in The Sarah Connor Chronicles. Cameron does not smile much - and everytime she is shown smiling, it is creepy as hell because the audience knows she's smiling to manipulate people. The sole exception to this is in the episode "Allison From Palmdale" where a scene showing her laughing and smiling is played as this trope - as Cameron is in her "Allison" persona and thinks she's human, and thus feels real emotions.
A male example of the trope is Aaron Hotchner in Criminal Minds. 99% of the time, he's so stoic and serious, his coworkers guiltily slink away if he catches them joking around with each other. Although, there are times he takes the role of the Deadpan Snarker and makes things ten times funnier when you realize the comment is coming from The Stoic. Once in a blue moon, though, he smiles at one of them or at his young son, and his whole face lights up with affection.
Jack: (who is dressed up in a mini version of what Hotch wears) He's not a real superhero
Hotch:He's not? I give up, then. Who are you supposed to be?
Jack: I'm you, Daddy.
Hotch: *Smiles* Wow. Let's go get some candy.
In Season 7 Hotch started dating again, finally getting over his wife's death, and you can see the other members of the team (especially Rossi) take absolute delight in seeing him smile.
Aeryn was a good example of this in the early days of Farscape. The rare occasions when she dropped her tough girl facade and gave a smile (usually as a result of something Crichton did) really brightened up the show.
From Merlin: the actors Bradley James (Arthur), Colin Morgan (Merlin) and Angel Coulby (Guinevere) have stunning smiles, all the more so because they are slightly imperfect (Bradley has crooked teeth, Colin's is a bit goofy, and Angel's is always slightly wistful), but when called upon to smile - especially at each other - you would swear that the sun shines a little brighter. See for yourself.
Also in the Super Sentai series, Sakura Nishihori/Bouken Pink started out as a super serious member of the SGS and can't smile ever. It takes one mission to finally make her smile for a boy she's taking care of. As her leader said, her smile is possessed by only hers alone, and during the climax of the series, she did declare that her smile was her greatest treasure.
Wonderfalls: Caroline Dhavernas is stunning as required per Hollywood law, but she manages to disguise the fact by looking permanently flabbergasted, resentful or miserable... until the very last scene.
Penny Parker (there's that name again!), the recurring character Teri Hatcher played on MacGyver, made her debut in an episode called... "Every Time She Smiles."
Dean Winchester of Supernatural. When he actually smiles (instead of flashing a Stepford Smile at whichever poor waitress etc happens to be nearby), it's a kiddish grin. And that kiddish grin just makes him all the much better, considering the amount of angst he gets.
Also Castiel. He smiles so rarely that when it does happen, it's a big event, and ◊.
Sam, too, to a degree. Good luck finding a real smile from either of the Winchesters after the end of season two, but early in the series you can find a few.◊ Usually when the boys are pranking each other.
Babylon 5's Ambassador Delenn, as played by Mira Furlan. In a season 5 episode, one character remarks that he'd claw his way through solid rock just to see that smile.
In Warehouse Thirteen Pete tells Myka to smile because he thinks she looks pretty when she smiles.
Pete "Smile. You're pretty when you smile"
Mrs. Frederic, who is stoic and communicates usually through Death Glares, her face absolutely lights up when visiting her grandson in his retirement home
Somehow averted in the 1995 version of Pride and Prejudice when Darcy states that Jane "smiles too much".
In light of Darcy's later actions as a result of his belief that Jane is a Gold Digger, the implication here is not so much that he thinks she doesn't have a nice smile but that he thinks she's using it insincerely.
In the final minutes of Parade's End, when Christopher Tietjens is dancing with Valentine after cutting ties with Sylvia, we see his face light up with one bright, beaming supernova of a smile that puts the sun to shame. Valentine visibly melts when she sees it.
In the Korean Rom Com drama You Are Beautiful, after a long day of getting lost in the city, when Hwang Tae-Kyung and Go Mi Nam who's really Go Mi Nyeo, posing as her twin brother while he's in America getting his plastic surgery fixed finally get back to the office Tae-Kyung, whose face is often wearing some gloomy, stoic, and/or grumpy look, smiles the most adorable little boy-like smile and makes Go Mi Nam's heart beat loud and hard. Naturally since this is out-of character for him, after the slow motion reveal his smile disappears just as fast as it came.
Every morning, Richard Castle brings Kate Beckett a cup of coffee just so he can see a smile on her face. Given what a serious and sombre character Beckett often is (particularly in the early seasons), and given the way Beckett's (and Stana Katic's) face lights up whenever she smiles, it's hard to blame him.
Sarah Walker (Yvonne Strahovski) is the consummate CIA spy, never smiling unless she's manipulating some mark, but her smile when she's around Chuck could power all of Burbank.
In The Walking Dead episode "Clear", Michonne's bright smile when she picks up a rainbow cat sculpture just transforms her whole face. Considering she spent nearly the entire season so far frowning and glaring (with times of fear, concern and worry), the effect is startling.
"The girl works at the store sweet Jane St. Clair / Was dazzled by her smile while I shopped there." - Barenaked Ladies, Jane.
That's a reference to the extended version of "Sweet Jane": "Heavenly wine and roses seem to whisper to her when he smiles; heavenly wine and roses seem to whisper to her when she smiles." Hear it here.
"and all her friends tell her / she's so pretty / but she'd be a whole lot prettier / if she smiled once in a while" - Shawn Mullins, Lullabye. (See The Pretender reference above.)
"Annie" by Safetysuit is about a 'plain Jane' who never liked what she saw in the mirror, but "she had a smile that could light up a room."
"Here in town you can tell he's been down for a while / But, my God, it's so beautiful when the boy smiles," - Anna Nalick, Breathe (2am)
As mentioned in the page quite, Nine Days' "Absolutely (Story of a Girl)" defines this trope and may well be the trope namer.
"When I See You Smile" by Bad English could also be this trope's theme song.
As seen in the quotes page, Van Morrison's "Jackie Wilson Said (I'm in Heaven When You Smile)" and The Stone Roses' "Standing Here".
Opeth's Mikael Akerfeldt. Not that he's normally unattractive, but when he smiles◊, he can be almost moe.
"Just The Way You Are" by Bruno Mars: "And when you smile, the whole world stops and stares for a while." Wow, that has to be one heck of a smile.
Well, it is a Silly Love Song, so it's possible the singer is intentionally using hyperbole to flatter the girl he's singing about.
And to think she's amazing just the way she is...
Celtic Woman's Orla Fallon. While she is hardly unattractive, she tends to fade into the shadows next to Lisa, Máiréad, Chloë, and Méav - but when her face lights up with a smile, there is no more beautiful woman on that stage, and you will feel the urge to smile right back at her. She's like a living ray of sunshine.
The human form of Amalion, a Third Circle demon in Exalted, to the point that she wears a veil to keep everyone she meets from falling head over heels in love with her if she cracks a smile.
In the comic opera The Firefly (no relation to Firefly), a waif wins a man over from his Disposable Fiancée by smiling at him.
Cyrano de Bergerac: Roxane is extremely beautiful even without smiling, but at Act I scene V, Cyrano mentions that her smile is akin to perfection. In act IV scene V, we know this is not an Informed Ability because of Roxane's answer to the Gascon cadets about how she passed enemy lines to smuggle them food:
Kaori Yae in the Tokimeki Memorial 2 game series does this, particulary after she has recovered from her Heroic BSOD. Lampshaded several times in the saga by the protagonist, such as in Dancing Summer Vacation, saying the following, as he notices how dazzling she is when she smiles:
Protagonist: "I have the feeling it's the first time I see her smiling, without being on her guards. It feels like she's a different person."
Raizze Haimer, the Emotionless Girl of Mitsumete Knight, has a special event and CG, where she has a rare beautiful smile, when the Asian is poisoned and she manages to save him. The game's CG Gallery names this CG "Mezurashii Egao" ("An unusual smiling face").
Throughout FEAR 3, the Point Man's face has been locked into moderate variations of either stoic determination or quiet fury, up until the end. During the Point Man's ending, while cradling his newborn sibling, he breaks into a smile before walking away.
KOS-MOS from Xenosaga, resident Robot Girl and Emotionless Girl, is already VERY attractive normally, but when she smiles at the end of Episode III, she's drop dead gorgeous.
Kyle Hyde from Hotel Dusk: Room 215. Normally, he is either frowning or poker faced. But when he breaks out one of these shiners...
In Shining Tears, if you've built up your relationship with the Ice Queen Blanc Neige, in the ending, she gave her first smile, as the complete sign of her defrosting.
Taiga spends a lot of his interactions with Muriel in Duel Savior Destiny wishing she'd lighten up a little. He's never really at ease near her due to her stoic attitude, so when she smiles and laughs at the end of Mia's route he's terrified and runs away.
An interesting subversion can be found in Phoenix's perpetually perky sidekick MayaFey. The first time Phoenix meets Maya, however, it is under rather miserable and unfortunate circumstances. When Maya at last flashes her signature cheerful grin, Phoenix notes that she looks like an entirely different person when she smiles.
Part of the fun of the Fate route of Fate/stay night is seeing the different reactions Shirou provokes out of Saber.
Riki: "Because I now know you can smile like that."
Hanako in Katawa Shoujo is both a fragile flower AND a shrinking violet because of her physical and mental scars caused by the fire that killed her parents. Her smile is recognized in-game as one of the most beautiful sights the protagonist has ever seen.
Shizune who normally has a very serious, almost bossy demeanor also seems to have a nice smile. At the very least, it's nice enough that Hisao tells her that she should smile more during her Act 1 ending.
Hatoful Boyfriend features stock images of birds as character portraits. There's just a single photo for each character, sometimes accessorized with Photoshop. The second game has a Shrine Visit in which the characters can be seen with multiexpressioned human portraits, "Like any normal human dating sim". One is of Nageki, who in the normal game sometimes resorts to just telling you how he feels at a given moment, because he's so opaque the protagonist can't tell. During the human-portrait shrine visit he mostly has the same sort of closed portrait, but at the end he does smile and thank the protagonist.
The Cyantian Chronicles: Celina is typically scowling or straight faced, but she has one of the warmest smiles of all the cast.
Whenever Microsoft Sam smiles in Looney Tunes Intro Bloopers it is actually quite adorable. Extra points if he does his signature laugh (Hahahahahahahahah soi soi soi soi rofl rofl rofl rofl) along with it.
Daria epitomizes this trope. She may look at the world with a jaundiced eye and a sharp tongue, but when she gives you a small smile when you've done something right or smart, then you will know it is the sincerest kind of thing she can give you. The show even used the Trope Namer song in one episode.
The opening sequence actually features this at the very end, though it's so zoomed in at that point you can only see her little smirk and not the rest of her face.
There was also Andrea the Goth when Daria and Jane discovered that she was worked as a wholesale club employee. She hid from them fearing they would mock her, only to find out they understand her situation and promise to keep quiet about it and she smiles realizing how she misunderstood how considerate they really are.
Somewhat appropriately, this extends to his Love Interest Mai as well- while she's conventionally good looking, you wouldn't really call her cute because of her perpetually sour expression. But on the rare occasions when she smiles... well, together with Zuko, there's a reason this◊ is one of the sweetest moments of the series.
put it this way: "Zuko's smile had widened, warm and genuine and little startled. He looked younger, disarmingly so, and Sokka found himself smiling back."
In Teen Titans, it's implied that Beast Boy feels this way toward Raven, as he puts immense effort into getting her to smile.
Subverted in The Grim Adventures Of Billy And Mandy, where they try to get Mandy to smile. They finally manage to, by the end of the episode, but it turns out that her smiling literally messes with the natural order of the universe and implodes reality itself, to the point that everything is reset and the three become The Powerpuff Girls.
In the animated adaptation of Heidi the sour housekeeper Madame Rottenmeier is astonished that Heidi could be taught to read, and Grandmamma turns to her and jokingly says "maybe we could teach you to smile". Madame does in spite of herself and it's lovely.
Superboy from Young Justice is easily the moodiest and least-positive member of the group. But on the off occasion, usually around M'gann or upon meeting Superman or his insane half-brother Match, he's revealed to have quite a lovely smile.