One True Love

"All of our young lives we search for someone to love. Someone that makes us complete. We choose partners and change partners. We dance to a song of heartbreak and hope. All the while wondering if somewhere, somehow, there's someone perfect who might be searching for us."

—Kevin Arnold, The Wonder Years

The most indomitable of story telling romantics is the idea of one utter perfect specimen who is your ultimate match. Perhaps they will have everything in common with you and share your dreams. Often Love At First Sight comes into play when the persion is beautiful. Destiny or The Power of Love are often invoked invoked to explain why that person is the way they are or how you met them.

Not to be confused with One True Pairing which is what the fans want to happen. This only applies to examples where it's made clear and deliberate in the work, regardless of what the editor happens to ship and regardless of how convinced you are.

Examples of One True Love include:

Anime and Manga

Inuyasha: A bit of a complicated example because Inuyasha's One True Love is Kikyou's soul. Inuyasha and Kikyou were deeply in love and were very close to living out their ideal married life, but this ended tragically when Naraku tricked the two into hating each other. Naraku fatally wounded Kikyou while disguised as Inuyasha, and expected Kikyou to take Inuyasha's life while using the jewel to heal herself. She could not bring herself to perform either out of love for Inuyasha and used her remaining strength to shoot a single sealing arrow to pin him to the Goshinboku tree. Kikyou was full of both bitter hatred and a very strong love towards Inuyasha at the time of her death. The Shikon Jewel became corrupted in response to her hatred while the love granted her a wish to see Iuyasha again. Naraku planned to use the tainted Jewel but Kikyou ordered Kaede to burn it with her body just before her death. Inuyasha and Kikyo's love transcended time and space as the desire to see Inuyasha again was the wish that the Shikon Jewel used to reincarnate Kikyou as Kagome and therefore reconstitute itself despite Kikyo's conscious decision to destroy the jewel with herself. Kagome becomes Kikyou's successor and although Kikyou was the woman Inuyasha "loved most in all the world", Inuyasha comes to realise that he is destined to be with Kagome (though they are different people, they are also one in the same). Inuyasha and Kikyou (as Kagome) finally get to live out their happily married lives together in the end like they had first planned those many years ago.
Naruto has been searching for the one person who would love him unconditionally. However, he was unable to reconcile this with the romantic implications of such a relationship, thanks to his emotionally-stunted childhood, which was deprived of such affection due to his status as the village pariah. He wanted it to be Sakura, because he saw her love for Sasuke, and being Sasuke's rival, sought that love for himself. The Last, however, makes it clear that the person he's really been searching for is Hinata, who has loved him from afar since childhood, but was always too shy to get close to him. Naruto was never able to notice her rather unsubtle feelings for him because he has never been able to understand romantic love. However, as the events of the movie progress and he finally begins to understand, he gradually comes to realize who the person he's meant to be with is, as he also understands his own feelings for her.
Yu-Gi-Oh! GX: Jaden and Yubel. In a past life as children, he promised to love her and only her forever. His feelings for her were so strong, they carried over through his reincarnation and memory loss: it only takes him a few minutes after remembering to reaffirm the vow and fuse their souls together for eternity. On Yubel's end, she loved him so much she voluntarily underwent a Painful Transformation into a monster just so she could protect him, and apparently waited thousands of years for him to be reincarnated. Her love was so strong it drove her to insanity, damned and redeemed her in succession.

Film - Live Action

The Princess Bride: Buttercup and Westley
Sleepless in Seattle: Lampshaded by Annie in one scene who says it is usually invoked as means of explaining all the romantic accidents. Played straight when she later goes along with the idea that Sam is the one anyway.
When Harry Met Sally...: Mentioned by Sally complains of a man she was interested in finding his perfect match. "He just met her... She's supposed to be his transitional person, she's not supposed to be the ONE."
Enchanted: Prince Edward and Giselle believe they are each other's. But it's Robert who awakens Giselle.

Live Action TV

The entire premise of How I Met Your Mother is based around Ted's search for his future wife, who has been shown to have the exact characteristics that Ted is looking for.
Raising Hope: Virginia and Burt have remained married for over 20 years, despite Maw Maw continually trying to break them up, because she wanted to boot out her freeloading family. Jimmy later narrates that "if two people are meant to be together, they will stay together no matter what tries to get in their way" - while deleting a text message on Sabrina's phone from her boyfriend, asking her to go to a restaurant, so he could take her out to eat instead.
Played for Laughs for an entire series of Peep Show, where Once an Episode Mark has decided that each and every girl he meets is the perfect woman for him.
Parodied in an episode of Friends: Phoebe asks Chandler if he believes in soul mates. He doesn't, he believes in falling in love, but not that there's one person who's perfect for you. She says "Good, because I just met Monica's!" The rest of the episode is him feeling increasingly threatened by this guy, until Monica explains she doesn't believe in One True Love either.

Monica: I don't think that you and I were destined to end up together. I think that we fell in love and work hard at our relationship. Some days we work really hard.

- This is rather odd as in their wedding vows Monica explicitly called Chandler 'my prince and my soulmate' and a previous episode had Chandler declaring they were "meant for each other". Many fans were irritated by the Continuity Snarl.
- Lampshaded by Word Of God in this quote:

Kevin Bright: What it says is that these two were destined to be together. That it doesn’t matter if they were skinny or large, or successful or not, that these two had something in fate that was meant to be.

Babylon 5 has John Sheridan and Delenn, despite the fact that she was his Second Love. The Power of Love between them has been enough for Sheridan to defy both death and torture and may in fact be responsible for winning two separate wars.
- From the same show: Jeffrey Sinclair and Catherine Sakai.

Literature

Bella and Edward in Twilight giving or attempting to sacrifice everything for the sake of each other. In their universe, it is deliberately invoked by the werewolves, who bond to a single soulmate for whom they always love.
"Lifemates" in the Liaden Universe are rare but when they do happen, that 'verse's science of psionics can objectively verify it. (It is eventually revealed that this is due to genetic spiritual meddling by Sufficiently Advanced Aliens in the past, and there's a damn good reason why it always seems to happen to the "dramliz" and not anyone else.)
In The Dilbert Future, Scott Adams Deconstructs the trope, pointing out that the One True Love is always within driving distance, and of the preferred gender, age, and species. And if the relationship doesn't work, the person usually will find their "True" One True Love under the same circumstances.

Video Games

In the infamous Sonic '06, the "Test of Love" states Sonic's soulmate choices, in which are narrowed down to Amy Rose and Princess Elise.

Webcomics

In Homestuck, this is apparently one of the cornerstones of the troll concept of romance. While trolls change lovers around much like humans do, trolls all believe that they have one perfect person (or in one case, two perfect people) for each of the four quadrants of their romance (romantic love, romantic hate, romantic guardianship and romantic mediation). This is called Troll Serendipity (in reference to that specific movie).

Western Animation

The Beatles episode "I Want To Hold Your Hand" has a scientist aboard a cruise ship, having captured an octopus whose unhappiness stems from being lovesick. The scientist lowers a diving bell into the ocean to gather some input on finding the octopus' true love. The Beatles have hidden inside the diving bell from their wild female fans, and under the sea they discover a similarly lovesick female octopus. Once back on ship, the boys buy the male octopus and throw him back into the ocean to be with the female octopus to live happily ever after.
It's all but stated that Numbuh 3 and Numbuh 4 are each other's True Love. Despite (or perhaps because of) their opposite personalities they complement each other very well. She's his Morality Pet and Living Emotional Crutch, doing unthinkable things for her sake and flipping out when something happens to her. She in turn brings happiness in him and only shows interest in him despite having many suitors behind her. Everyone Can See It In-Universe and out, and both Cannot Spit It Out until the finale, when it's revealed they got married.
Per the words of Craig Bartlett, Hey Arnold!'s Arnold and Helga are this to each other. Most of the show's episodes focuses on their relationship, and Bartlett stated they'll eventually become an item and marry as adults. In-series, it's not until The Jungle Movie they finally hook up.