9. Wally West and Jesse Quick
The relationship between Wally West and Jesse Quick was more divisive than it should have been and that's just down to the fact that the show never really gave them anything of importance as a couple. It was nice watching them grow closer but that's really where the development of their partnership stopped. It's a shame because there was real potential there, and some good chemistry between Keiynan Lonsdale and Violent Beane.
However, when the story became all about Wally's desire to develop powers (and Jesse's actual development of powers), the relationship they were forming was essentially dropped. It wasn't put on the backburner, it was dropped. Within a season, Wally and Jesse were both no longer part of the show, and just a few seasons later, Jesse was unceremoniously killed off off-screen during the Crisis.
This was a relationship that should have been easy to get right, but the stars didn't align for them. Wally and Jesse are comic book royalty and there was excitement to see them together on the series, but between the bland way their relationship was handled on-screen to the disrespectful treatment of the characters off-screen, it ended up being a major missed opportunity for The Flash.