2. Chester P. Runk and Allegra Garcia
Sometimes you can take two perfectly fine characters and saddle them with the most basic of romance tropes, only to instantly turn fans against them. That's exactly what happened to Chester P. Runk and Allegra Garcia on The Flash season 9.
Chester was a fun and enjoyable addition to Team Flash in season 6 while Allegra had the potential to be both but was never quite given the chance to shine. Pairing these two off in a romance prevented either one from ever shining again, as the storyline gave them absolutely nothing worth caring about when the show was hurtling towards its conclusion.
A romance between Chester and Allegra could have been fun. Heck, their scenes out-of-context are pretty cute and there was definitely chemistry between them. Unfortunately, the show decided to play the long game with what was a really obvious storyline, dragging it out for nearly two seasons when literally none of the fans cared enough to pay attention to it. It ate an endless amount of screentime, particularly in season 9 - which was the show's final season and should have focused on storylines that actually mattered.
Fans hated this storyline for that reason as it stole valuable minutes of screentime from the show's main characters and every single beat that it hit was full of relationship tropes that we've all seen before; tropes that came with no tension, no stakes, and tropes that were easily resolved. What a waste.