7. "Wildest Dreams" (season 9, episode 7)
As a major fan of Supergirl and of Dreamer, it pains me to see this episode land on the list of The Flash's most disliked episodes, but it's also hard to argue with the logic of placing it there.
"Wildest Dreams" was one of the episodes we knew most about heading into The Flash's final season as it saw Supergirl's Nia Nal team up with Iris West. Unfortunately, the episode wasn't anywhere near as fun as it sounded, with the bulk of it being around Nia's inability to understand what her most recent dream meant. And, if you've watched Supergirl, you'll know that it's failure to showcase Nia's potential through plots like this was one of the biggest issues of its final few seasons. We didn't need to see it happen all over again on The Flash.
However, the biggest problem that fans had with this one was that it was one of the many wasted episodes in The Flash's final season. It's not that it wasn't great to see Nia again (because it was); it's that it sidelined Iris in the storyline, sidelined Barry outside of it, and carried no real emotional weight or stakes, nor did it have a lasting impact on the story.
It was just there, taking up a spot in the final season. And as you'll come to see from this list, it wasn't the only one to do that.