At the beginning of the novel, Yoshitoki has a crush on Noriko, who likes Shuya, who likes his upperclassman Kazumi Shinitani, who has a boyfriend. There's also all the other girls who are in love with Shuya.
Chisato Matsui turns out to have a crush on Shinji Mimura, which is only revealed after he's killed.
Hiroki Sugimura turns out to be in love with Kayoko Kotohiki, who in turn is in love with an older boy at her tea ceremony training school. Meanwhile, Hiroki's childhood friend Chigusa Takako seems to be in love with him.
There's also the complicated situation between Yoshimi Yahagi and Yoji Kuramoto. They had been dating, and while Yoshimi was deeply in love with Yoji, when they run into each other during the Program, Yoji claims he was only with her because he thought she was easy, and yells at her stay away from him while holding her at gunpoint. After Yoshimi stops fighting him, reaffirms her love for him, and allows him to shoot her, he stops and tears up, seemingly actually returning her affections. Yoji's feelings are left unclear, however, as Mitsuko shows up and kills them both right after.
By the end, the only aversions are Shuya seeming to reciprocate Noriko's feelings, and Sakura Ogawa and Kazuhiko Yamamoto, who are dating, but kill themselves when the Program starts. Everyone else's love remains unrequited.
Dr. Tijou has a crush on Baba Eddie, who has a steady boyfriend already.
In Far from the Madding Crowd by Thomas Hardy, three men propose marriage to Bathsheba Everdene. Both of the first two men love her, but she refuses them both. She marries the third, then discovers that he's in love with another woman.
Steinbeck's entire East of Eden revolves around this trope, but with love in the general sense: father/son and brother/brother in addition to romance.
Bret Easton Ellis' The Rules of Attraction exaggerates this trope to the point of comedic ridiculousness, using a narrative which transfers constantly between ten or eleven promiscuous arts college students (though there are 3 central protagonists), all of whom have unrequited feelings for one another and/or more minor characters.
In Victor Hugo's The Hunchback of Notre Dame, deformed bellringer Quasimodo, increasingly unstable priest Claude Frollo, and lazy poet Pierre Gringoire are all in love with La Esmeralda (Gringoire even ends up married to her, though that doesn't mean he gets to touch her), who loves the soldier Phoebus, who's engaged to another woman but fine with boning Esmeralda on the side so long as it doesn't inconvenience him in any way and he doesn't have to acknowledge her in public.
Hugo likes this one. In 1400 pages, Les Misérables features one reciprocated romance with Cosette and Marius. The rest all end horribly. Fantine is ditched for a laugh by her feckless bourgeois lover Tholomyes—she becomes a prostitute to support their illegitimate daughter Cosette, gets sick and dies. Eponine develops a huge crush on Marius, but he doesn't even notice until she takes a bullet for him at the barricade and bleeds out in his arms. And in the (sort of) platonic sense, Grantaire doesn't care about the revolution but is deeply devoted to Enjolras, while Enjolras is too devoted to his glorious cause to put up with Grantaire's cynicism—until the National Guard comes to shoot Enjolras, and Grantaire is allowed to die at his side. Madame Thenardier is also stated to be deeply devoted to her husband while he is quite content to let her rot in jail and is completely unmoved when he hears she is dead. She hadn't ever done anything to earn his scorn, he simply never cared for his family in the first place. Their two youngest children would also count in a familial sense, being abandoned as children, forced to walk the street looking for parents they know nothing about.
In the book Death Star, twenty years after the events of the Medstar Duology, the one character to feature in all three books is a surgeon who went from a teenaged prodigy to a cynical middle-aged man who's been permanently drafted. In Death Star he reflects that he still doesn't know if he loved Bariss Offee, a Jedi he'd known during the Clone Wars. Whether he did or didn't, her death hit him hard, and he thinks of her often. As the Medstar Duology featured Bariss as one of the viewpoint characters, the readers know that she was totally unaware—she'd known about another character who thought he loved her, and she'd spent a lot of time with the prodigy, but she never knew.
The Mad King, Aerys, would have rather wed Joanna Lannister than his sister, which combined with his increasing madness as he grew older made him come to hate his childhood friend, Joanna's husband Tywin Lannister out of jealousy.
I Capture the Castle: Stephen is in love with Cassandra, who's in love with Simon, who's in love with Rose, who's in love with Neil.
Ginny likes Harry, who has a crush on Cho Chang, who is dating Cedric Diggory. After Cedric dies, Harry and Cho try to date. It fails miserably. By this point in time, Ginny has moved onto Dean Thomas… just in time for Harry to realize that he likes Ginny after all. Romilda Vane also tries to feed Harry a love potion.
Severus Snape has always loved Lily Evans. Lily eventually marries James Potter, who also has an unrequited crush on her for awhile.
Bellatrix quite clearly loves Voldemort, who isn't really capable of feeling or comprehending love.
Hermione and Ron love one another, but each has a few throw-away Love Interests; Hermione (along with the rest of the girls at school) has a crush on Gilderoy Lockhart, goes to the Yule Ball with Viktor Krum (who retains feelings for her after the fact, as well as expresses interest in Ginny at the wedding), and takes Cormac McLaggen as her date to the Slug Club party specifically to annoy Ron. Ron, on the other hand, has a thing for Fleur Delacour (who marries his older brother, Bill) and Madame Rosmerta, and dates the thoroughly obnoxious Lavender Brown to get back at Hermione for dating Krum. He also hilariously becomes infatuated with Romilda Vane after accidentally ingesting the love potions she put in Harry's chocolates.
Per Word of Gay, Dumbledore once had a crush on Grindelwald. It was unrequited, but that didn't keep the latter from exploiting it for their own gain.
Wicked: Boq has a long-time crush on Glinda, who seems to be in love with Elphaba, who may or may not return those feelings. Later on, Elphaba is in love with Fiyero, who is altogether confused about the concept of love (he was betrothed at the age of seven, so that's understandable), but seems pretty convinced that he feels the same way. His widow, Sarima, doesn't care about him too much, but seems to be furious when confronted with the fact that he was cheating on her (but she thinks it was with Glinda).
Twilight: Jessica likes Mike, Lauren likes Tyler but both boys (along with Eric and all the male population of Forks) are interested in Bella. Bella couldn't care less about them and falls for Edward, the only one who seemed to hate her at first.
And of course Jacob also counts, as he also falls in love with Bella, but she only considers him a friend.
Leah was engaged to Sam, until he imprinted on Emily. Losing her fiancé and best friend at once has naturally left Leah kind of bitter.
Song at Dawn: Arnaud is in love with Estela who is in love with Dragonetz who is in a relationship with Emerganda who is in love with him after he falls for Estela.
Wings of Fire: Fatespeaker loves Starflight, who loves Sunny, who only thinks of him as a brother. The second arc is much worse; Turtle may or may not have a crush on Kinkajou, who has a crush on Winter, who is in love with Moonwatcher. Qibli is also shown to like Moon, and it's unclear whether she likes either of them back. Qilbi also has some Ho Yay with Winter. Umber has a crush on Qilbi.
Celebrimbor loves Galadriel. Galadriel loves Celeborn. This is one of the few cases of unrequited love in Silmarillion that doesn't result in the death of at least one person involved. Well, sort of - Celebrimbor dies, but because Sauron deceived him, not because of his love for Galadriel.
In Sarah Gailey's River of Teeth, the keen-eyed but impulsive cardsharp Cal is in love with the resident Emotionless Girl contract killer Adelia. Adelia does not care for men, in general or in his specific case.