Kyon and Haruhi in Haruhi Suzumiya. This is rather difficult to tell, given Kyon is an Unreliable Narrator and half the time he's lying to himself, the reader, or other people. This has created a large Will They or Won't They? among the fanbase, considering Kyon's being especially vague and there being a larger ulterior motive behind everything he does that would normally be counted as romantic feelings. However, given the 95% chance he's probably getting with her, Kyon refuses to acknowledge the relationship and says nothing on the subject. This doesn't stop all of his classmates from assuming that he's Haruhi's first love interest in a while, adding to Kyon's reputation for liking "weird girls"...
7 Seeds: Semimaru hides his feelings for Natsu, but Hibari, Chimaki, Arashi, Hotaru and even Matsuri by now have done slants at how obviously Semimaru likes Natsu. Natsu doesn't notice herself.
In Delicious in Dungeon, everyone in Team Touden, except Laios, was aware that Shuro had feelings for his sister, Falin. He's a little embarrassed to learn about it.
Detective Conan: Shinichi and Ran are considered already married by their friends. It is that obvious. Except for the fact that she blatantly tells "Conan" about her crush in the first episode. Still Selective Obliviousness on his part though.
Otani and Risa in Lovely Complex are shipped mercilessly by their friends well before they get into a relationship.
Dragon Ball: Tien and Launch get a bit of this, Launch is openly attracted to Tien while he's either oblivious or just shy. Even pervy pig Oolong makes note of it causing Tien and Launch to blush. Launch is unfortunately absent for most of Dragon Ball Z, but the reason Tien moves around so much is because it's hard to keep up the fighting-monk demeanor when there's beautiful woman chasing you around. Apparently Tien worked it out with Launch... Somehow.
Dragon Ball Z: Gohan and Videl got a lot of this, especially by Chi-Chi and Krillin. Hell even Goku brings it up! Naturally, Gohan often yelled the classic "She Is Not My Girlfriend" to shut them up. Even at the victory party, everyone wants them to dance together, and they do so shyly.
Dragon Ball Super: Future Trunks and Future Mai are so clearly this that both Goku and Vegeta see it. Just before they fight Goku Black, Vegeta encourages Trunks to survive and build a better future with his "girlfriend." This gets both Trunks and Mai to blush.
The Mazinger series:
Great Mazinger: Tetsuya and Jun. Unfortunately, Tetsuya was even more oblivious. How oblivious? He thought Jun would replace him with Kouji in spite of: Jun barely knew Kouji; and she had spend the whole series trying Tetsuya noticed her.
UFO Robo Grendizer: Duke averted this trope with Hikaru and with Rubina. However, Kouji was absolutely oblivious to the feelings of Maria. Of course, he was the only one.
K manages to combine this with Foe Romantic Subtext - Saruhiko defected from HOMRA to Scepter 4, and Misaki hates him as a traitor... but the rest of HOMRA are mostly cool with his decision, since they realize it was just what was best for him. Kusanagi in particular always responds to Misaki's rants about hating Saruhiko with a "Sure, keep telling yourself that," tone. Misaki finally comes around at the end of season 2.
Alphonse Elric gets some mileage out of teasing Ed and Winry about their obvious feelings for each other. Riza also makes Ed have a rather spectacular Spit Take on poor Black Hayate (resulting in a Funny Moment) when she addresses his love for Winry. As a note to just how obvious Ed and Winry are, not only can Al and Riza see it, but Hughes assumes they are in a relationship already. Heck, Bradley and Kimblee seem to have figured it out, which should tell you pretty much everything you need to know about these two. When the literal personification of Wrath and the genocidal Mad Bomber have picked up on it, it's pretty obvious that yes, everyone can see it.
Meanwhile, the cast also ships Roy/Riza. This is an odd case, however. It isn't quite so much that they're in denial about how they feel for each other, but rather that they're not allowing themselves to act on it for other reasons. (He's her direct superior in the military, and there's an indication that an anti-fraternization law is largely what's keeping them apart.) Everyone can still see it, though, and that unfortunately includes Bradley and his cohorts. In this series, No Loves Intersect, but everyone can see all of them. And ships them.
This characterizes Cosmo's and Tails' romance in Sonic X—at least in the 4Kids dub. The Japanese version had them be more explicit about it.
Ouran High School Host Club has Wholesome Crossdresser Haruhi and her obvious feelings for the Host Club's King, Tamaki, and vice versa. The entire club, being in on the crossdresser thing, takes note of the impending coupledom before the characters in question even begin to entertain the idea. Of course, it doesn't help that Tamaki used to see himself as Haruhi's second father, and have we mentioned the whole crossdressing thing yet?
Konoka and Setsuna. Everyone who sees them together immediately assumes the obvious. Including Negi, who's the epitome of Oblivious to Love. Konoka is probably aware of this, but Setsuna takes a lot longer to catch on, finally getting it when they finally make a pactio and the kiss lasts forever...
Nagi and Arika. Every single Ala Rubra member can see it.
Kyohei and Sunako in The Wallflower. By the manga's second half, the main plot question has gone from "Will Sunako ever become a lady?" to "Will these two dense idiots ever realize they're in love with each other?" Even their housemates are getting tired of it.
Hinagiku is clueless as to how everyone around her can see her crush. But then, this is a series where pretty much the entire cast is suffering from an Oblivious to Love epidemic.
Hayate's crush on Hinagiku is even worse for being clear to everyone. He even tries to tell others that she couldn't possibly like him. There's also the incident in the elevator in the student council clock tower where Hayate completely failed to hide it from a barely-disguised Maria.
Miki, Maria, Nishizawa, and possibly even Athena make it clear that they know about Hayate and Hinagiku's mutual crush. Even Nagi seems to know about it and has expressed both suspicion and jealousy at various points in the manga. Oddly enough, she let Hinagiku go on a dinner date with Hayate to show his appreciation for all her help in Greece but refuses when Ayumu tries to do the same. Most likely, she guessed that nothing would happen. She was waaaayy off.
Played with in Kimi ni Todoke, when everyone jokes about Sawako and Kazehaya liking each other, only to have unknowingly spoken the truth.
Throughout one of the Drama CDs, it's rather explicit that the nation-tans see America and England (both real heavy on the Tsundere traits) in this way. In the last track, everyone else stranded on the island hears an exchange between England and America at night.
Lithuania: R-Russia! Everyone was pretending not to hear anything! England: W-what? Everyone's awake, after all? France: That's because you were having a lover's quarrel here. England: Don't say it was a lover's quarrel! Japan: Well, it's not like we were peeping and such...
Even the dubbers can see it: one episode of the dub has France asking America and England if they're done "releasing sexual tensions".
Asuka calls Hikari on her Belligerent Sexual Tension with Toji, saying that everyone can see it except for the "Three Stooges" (Toji, Kensuke and Shinji).
Elie and Haru in Rave Master, to such an extent that the Big Bad—who spends, at best, a total of one hour in contact with them over the course of the series—feels the need to tease them about how slowly they've been taking their relationship.
The Yamamoto sisters in Hanamaru Kindergarten have Guiness World Record levels of obliviousness to love. This despite the fact that, in the older Nanako's case, even kindergartners realize Tsuchida is in love with her. Making it even funnier, Nanako is blatantly aware that Hanamaru-sensei has feelings for Mayumi, while Mayumi is likewise aware of Tsuchida's feelings for Nanako, and one episode ends with both of them expressing pity for the guys having to deal with such a clueless girl.
Mikado and Anri from Durarara!!. It's noted in the Light Novels that one of the only reasons people don't ask Anri out is that they just kind of assume she and Mikado are already dating.
Deconstructed and later reconstructed in Berserk. Before Guts leaves the Band of the Hawk, Judeau pushes the would-be relationship between Guts and Casca as a way to persuade him to stay (or rather, to get him to take her with him), saying that they would be good together. However, Guts pulls the "Just Friends" routine, because he believes that Casca only has eyes for Griffith and that he was no competition for him. He couldn't have been more wrong, and only realizes this after he returns to the Hawks a year later when everything turned to shit but luckily he and Casca get to share some joys in their new found relationship upon his return before shit turns into deeper shit.
Everyone can see that Mikoto is completely and utterly smitten with Touma. Even Kuroko can see it, much to her chagrin. Even her clones can see it, especially #10032 (aka Misaka Imouto) who happens to feel the same way and openly declares that she'll "take a different route than the original" before glomping onto his arm. Touma (probably) hasn't noticed, but that's justified because he has amnesia but refuses to tell anyone.
On the Magic Side, everyone can see Kaori Kanzaki has feelings for Touma. Be it Motoharu, her comrades in the Amakusa Catholic Church (including her kouhai Itsuwa, who also has feelings for Touma), and thanks to rumor about half of England.
In Mobile Suit Gundam Wing, about half the cast (primarily Duo, Trowa, Quatre, and Dorothy) recognizes the really close bond that Heero and Relena share, which has a really good chance to become mutual love, and do their best to help things along. Unlike most instances of the trope, the two do get along quite well normally, but Relena (whose Image Songs outright say she's in love) won't press the issue because she doesn't want to force his hand and/or destroy the closeness they already have, and is just happy knowing that he's looking out for her. Not to mention, the girl is already very devoted to her cause for peace as specially after being the Queen of the World and becoming Vice-Minister. Heero's own feelings are left more in the dark, but the common assumption is that he doesn't feel worthy of her, or possibly that his unusual upbringing means he doesn't quite understand his feelings for her. In any way, he does care for her and will help her when she needs it.
A good part of the Mawaru-Penguindrum cast has at least commented once on Ringo Oginome and Shouma Takakura's odd relationship. Shoma's siblings, Ringo's mother, Ringo's crush as well as said crush's own girlfriend, Kanba's Clingy Jealous Girl
Ranma ½: When it comes to Ranma and Akane, everybody but the rivals and Ranma's harem can see it. The entire school thinks the two of them will get together, because they argue so much, and they enjoy teasing them both about it.
Boy: Ooo, a lovers' quarrel! Girl: Practically married already.
In Sword Art Online: It takes people about five seconds of watching the pair interact to realize Asuna and Kirito are going to end up together. This can be heart-rending when the ones realizing this are girls who have fallen for Kirito.
Revy and Rock in Black Lagoon. It's so obvious that Eda constantly brings it up with Revy, and Bao flat out asks Rock if the two of them had a fight just because he happened to not be with her when Bao saw him.
Ryuuji and Taiga in Tora Dora spend a significant chunk of the series explaining to others how they're not going out and don't have feelings for each other. Yet somehow, nobody is surprised when the two of them show up in the middle of the night, running away to elope.
Horimiya's main characters, Hori and Miyamura, both deny feelings for each other, although Hori's reasons are more that of a typical Tsundere and Miyamura's are more that of feeling unworthy. Of course, everyone else has a hard time believing it, even Hori's Dogged Nice Guy Ishikawa, though it does practically send him into a Heroic BSoD every time.
Played with in Bokura no Kiseki. Everyone living in the castle in Zerestria could tell that Veronica and Glen were a couple. But when Veronica's Arranged Marriage is decided on and Glen's fellow squires all gather to comfort him, in surprise he tells them that there was absolutely nothing between him and Veronica beyond a close friendship. The protagonist Harusumi, who is Veronica's reincarnation, confirms this. However, a deeper look at their feelings shows it's likely they were merely both Oblivious to Love, thus playing this trope straight.
Averted for the most part in Sakura Trick, as everyone comments on how Yuu and Haruka are a little too clingy for friends, but no one seems to notice the two are in a Schoolgirl Lesbian relationship despite them being very bad at hiding it.
From the series Place to Place, Tsumiki is very obviously crushing on Io and the only one who doesn't seem to notice it is Io himself. All the other characters ship them wholeheartedly and their reaction to learning they were compatible from a magazine's compatibility test basically amounted to "Well, duh!"
It doesn't take long for Noa to form a close bond with her partner, Asuma. Nor does it take long for their squadmates to notice they have chemistry with each other. Even Hana realized it, during her brief stint with SVU2, which causes her to forego her own crush on Noa, so she could be together with Asmua. Predictably, Noa and Asuma had no idea what gave her that impression and deny it. Though the last 2-3 episodes of The New Filesimply otherwise.
You're Under Arrest!: Similar to the Patlabor example, it's no secret to anyone at Bokutou Precinct that Miyuki and Nakajima are crazy about each other, except they're both too shy to say anything. Natusmi and the others have even gone as far as to set them up on dates to try to get things going between them, but to no availnote The closest they've ever gotten was when Nakajima nearly proposed to Miyuki during season 2 ("Fast & Furious"), 'til he chickened out at the last second. Miyuki understood what he was trying to say and tells him that she'll wait for him.
Highschool of the Dead: It takes them awhile to get there, but by Chapter 28, it's clear that Takashi and the others can see how things are shaping up between Hirano and Saya. In that chapter, when Saya admits she'd been watching him so she could learn to fight like him, it leads to a tender moment where they exchange guns, which cause Alice to wave a pair of flags while the others smile knowingly.
Amongst all the Ship Tease, Shikamaru and Temari were fairly obvious from the get-go, with Naruto even assuming they were on a date after he first came back to the village after the Time Skip. They are married with a son in the epilogue.
After the Invasion of Pain arc, nearly everyone in Konoha and all of their friends knew Hinata was in love with Naruto... except for Naruto himself, due to his emotionally-stunted childhood. In The Last: Naruto the Movie, when Naruto finally realizes he's in love with Hinata, he is incredibly obvious with his feelings, which makes Hinata's oblivious status painfully ironic and causes many characters to give them knowing looks. Considering Kiba's throwaway comment to him in Chapter 611—which Naruto did not deny nor poke fun at—and Sakura's knowing smile at his reveal in The Last, teasing Naruto's feelings for Hinata started long before the movie, and it lends credence to the implication that he had been in love with her since the middle of Part II. They are married with two children at the end of the movie and in the epilogue.
Bisca and Alzack, minor characters who are the only ones unaware of the fact that their feelings are mutual.
Maken-ki!: It's a well-known fact at Tenbi Academy that Haruko is in love with Takeru. Mainly because, as shown in Chapters 12 and 37, she's so bad at hiding it that it's become a Running Gag. The only one she's fooling is herself.
Ash and Misty. Despite Brock doing his best to point out they clearly have feeling for each other. Every big argument Ash has with Misty is akin to an old married couple according to people who see them at it. It was only when Ash left for Hoenn that Misty realized why she feels so sad. When they meet again Misty is happy to see Ash but doesn't tell him why.
Serena in the XY series finds out, much to her embarrassment, that her feelings for Ash are very obvious to just about everyone (the boys apparently don't see it, including Ash himself) they meet in their adventure. So far, Alexa, Miette, Bonnie, Shauna and Nini have noticed it; Miette even pretends to be a romantic rival to help push Serena into confessing, while Shauna rolled her eyes when she sees that Serena wants to keep Ash in the dark about it.
Magical Girl Lyrical Nanoha Strikers: Nanoha and Vivio were a non-romantic example. Everyone knew from day one that Nanoha was the perfect person to adopt Vivio despite her various protests.
In addition, Mari sees how Vivio interacts with Fate and Nanoha, and immediately says "oh, she's their daughter."
Kaguya-sama: Love Is War: Kaguya and Shirogane are so obvious about their feelings that are many rumors flying around that they're dating. Chapter 50 shows the audience just what everyone else can see; in the eyes of a neutral party (in this case, Kashiwagi), all of the serious, mind-wracking battles that Shirogane and Kaguya go through and their extreme efforts to deny their feelings to themselves and each other end up looking like awkward flirting and thinly veiled confessions.
During the first chapter (and subsequent flashbacks that take place before the events of that chapter), nearly everyone at Hiryuu Castle could tell that the titular Princess Yona has a pretty huge crush on her cousin Suwon, save for King Il.
It's very much clear to most other characters that Childhood Friend Hak harbours some serious unsaid feelings toward Yona, despite that he tries his best to hide it. Jaeha in particular takes it upon himself to tease him about it whenever possible. Later, when Yona grows to fall for him as well; many characters figure it out long before she does.
So, I Can't Play H!: Though Lisara stubbornly refuses to admit her feelings for Ryosuke, she isn't fooling anyone. Mina silently wishes her well, even though it means losing Ryosuke to another girl, whereas Quele's determined to win his affections before her cousin, Lisara, does. It doesn't take long for Ilia to see it either, as she's the only one with no interest in him.
Lisara:(mumbling into pillow) "There's no way I'd ever fall in love with someone like him."
Ilia:(back turned) "You always were a terrible liar...."
Yu-Gi-Oh! ARC-V: Yuya and Yuzu are a shocking example considering what the franchise is known for. But nearly everyone that knows the both of them knows that something is going on between them. The biggest example is when Tokumatsu who Yuya befriended when he was separated from Yuzu and didn't mention that his reason for arrive quickly asked Yuzu is she was his girlfriend after watching her determination.
City Hunter: Numbers of men and women give up chasing romance with the main characters simply because they notice Kaori has feelings for Ryo or/and vice-versa. Ryo and Kaori, of course, are completely clueless.
The titular characters in Mishiro-san to Yamada-kun. They've been friends since childhood, share a lot of the same interests (namely the same video games), are extremely close (to the point that Mishiro is completely fine with staying over at Yamada's house even without his parents being there), and like to tease each other. To the point that, when Mishiro tells her parents that she has a boyfriend now, her father immediately (and correctly) assumes it has to be Yamada, and her mother remarks that she thought the two had been sleeping together for a while now. The only member of her family to be surprised is her younger sister, and even then she's more surprised that the two actually got together, not that the two like each other. To a lesser extent, their friends Mawari and Susuki, though their relationship receives less focus than Mishiro and Yamada's