Clarke and Lexa - The 100
Whether or not Clarke and Lexa knew their love wouldn't last isn't the point. The point is anything is possible in TV land, and the possibilities these two could have had a lasting romance is one that will haunt us forever.
Wes and Laurel - How to Get Away with Murder
While there are many who weren't too keen on the Wes and Laurel hookup, they had been close friends before they ended up in the bedroom together. Whether their relationship would have lasted is anyone's guess, but Wes' murder put all the speculation on hold.
Butch and Tabitha - Gotham
Butch and Tabitha always had an odd relationship, but it was a romance they both enjoyed. Butch went through many trials and tribulations and just when he fixed himself in anticipation of a happy life with Tabitha, Penguin shot him down. Nothing lasts forever, especially on Gotham.
Jack and Rebecca - This is Us
Jack and Rebecca had one of the greatest love stories on television. We all knew he had died, we just didn't know how. When it was finally revealed, we were as devastated as Rebecca and the kids. Jack and Rebecca deserved better, and so did we.
Regina and Robin Hood - Once Upon a Time
Regina and Robin's romance didn't have a happily ever after like all fairy tale romances should. It doesn't matter that Robin sacrificed himself to save the woman he loved, all that matters is that their future happiness was crushed -- and so was ours.
Willow and Tara - Buffy the Vampire Slayer
This romance was a slow burn evolving from a deep friendship into one of true love. These two should have had a happy life together, even with ups and downs found in any relationship, but their future was stripped when a stray bullet killed Tara. It was a cheap shot (no pun intended) that no one in the Buffy fandom deserved.