6. Joe West and Cecile Horton
It's wild to think that we're putting anything to do with the great Joe West on this list. The character is one of The Flash's strongest characters, but unfortunately one that the show stopped knowing how to use around the halfway mark of its run. His burgeoning relationship with Cecile was an easy one to root for in the beginning, but then the show just stopped caring about giving them meaningful storylines, and the audience stopped caring about them in general.
Joe and Cecile's relationship was like a footnote in the show's back half. They shared so many scenes together, but there was rarely anything of importance to their partnership going on in them. And when they did have meaningful conversations, it was usually about ridiculous things like Cecile's random desire to become a superhero in the final season, essentially leaving her family behind in a different city.
Honestly, Joe and Cecile probably wouldn't have ended up on this list of it wasn't for season 9. While the fans had grown weary of Cecile long before that, her relationship with Joe was one of the few times she was at her strongest. But the appalling decision to write Joe out by moving him to a different city and having Cecile apparently devote the rest of her life to becoming a superhero (but also visiting her family on the weekends because crime apparently stops on the weekends) was so reviled by audiences that it elevated their reaction to the couple from indifferent to outraged.
Also, the pair have a daughter named Jenna who was literally absent from the show for about four seasons. Remember her? Neither do they, apparently.