Matthew and Mary, Downton Abbey
After years of back-and-forth on Downton Abbey, spanning a world war and broken engagements to other people on both sides, Cousin Matthew and Lady Mary finally married AND had a baby. Mere minutes after seeing his son for the first time, Matthew was in a fatal car crash and died (as a result of Matthew's portrayer Dan Stevens electing to leave the series). None of Mary's potential love interests in the years since Matthew's death have come remotely close to recapturing that Mary-Matthew spark that drove the whole show forward.
Sun and Jin, Lost
Sun and Jin had a complicated relationship. In fact, in the first season of Lost, it appeared that Jin was actually the epitome of a terrible, abusive husband to Sun. As more and more of each character's backstory was revealed, the quality of their relationship and the love between them deepened until they finally fixed their relationship in the present day. Sun and Jin were literally separately in space and time and somehow managed to find their way back to one another... only for them both to tragically die on the island when Jin refused to abandon a drowning Sun, leaving their baby girl an orphan. Romantic gesture on Jin's part? Sure. Deeply ill-advised? Um, yes.
Clarke and Finn, The 100
Though Clarke and Finn had plenty of chemistry right off the bat, the relationship didn't lack for drama. For starters, the two had their "meet cute" (not so cute) aboard a drop ship surrounded by 98 other teenagers, barrelling down towards an apparently-inhabitable futuristic Earth. Their relationship grew organically, until it was disrupted by the arrival of Finn's girlfriend. Though they eventually managed to find their way back to one another, that reunion was cut tragically short when Clarke ended up having to mercy-kill Finn to spare him a gruesome, protracted execution by the Grounders. One scalpel to the stomach and "Flarke" was no more.
Tate and Violet, American Horror Story
Despite Tate being an all-around terrible person (er, rather, terrible ghost), the Tate/Violet coupling (fittingly known as "Violate") was hugely popular for fans of American Horror Story's first incarnation, Murder House. Tate was deeply troubled and did many horrible things, but his insistent love for Violet shone through as this weirdly pure thing amidst all the darkness around them. And while it seemed like they might have been united in death, Violet finally put her foot down and ordered Tate out of her sight forever once his many hideous crimes were unveiled.
Cole and Phoebe, Charmed
Oh, Cole. He tried so hard to not be evil, for so long. Cole and Phoebe were a great example of star-crossed lovers. She, an uber-powerful witch (alongside her sisters, the Halliwell witches) and he, a high-level half-demon/half-human of the underworld. He was originally sent to seduce and kill her, but ended up legitimately falling in love with her. Their romance was chaotic, fraught with drama, and very passionate, culminating in their eventual marriage. Unfortunately, their happiness didn't last for very long, as Cole eventually wound up becoming The Source of All Evil and turning Phoebe temporarily evil in the process. After vanquishing him a few times, Cole finally stayed (mostly) dead and Phoebe moved on romantically.