4. "Rayo de Luz" (season 7, episode 14)
"Rayo de Luz" was originally considered the worst ever episode of The Flash, achieving a low score of just 4.8 on IMDb. I'm not sure it deserved that honor, but I can see why it was granted that title, because it was one of the biggest examples of everything wrong with the modern version of The Flash.
The episode's biggest problem was that it focused solely on Allegra, a new character who had only been introduced in the previous season. There was certainly potential there and audiences were growing to like her, but placing the spotlight on her at the expense of the main characters wasn't going to sit well with fans. And - spoiler alert! - it didn't.
The episode revolved around the return of Allegra's cousin Ultraviolet, another underdeveloped character who fans just didn't care about. But Allegra was convinced she could be changed, and set out to help her, eventually realiziing just how powerful she was too. And yes, Allegra has Captain Marvel-level powers; except The Flash never once treated that like the interesting topic of discussion that it should have been.
"Rayo de Luz" had its moments and it was nice to see more of a focus on Allegra's serious side, but this firmly established the interlude status quo that required at least one of these time-waster episodes per season. None of them were well-received and yet they persisted. But hey, at least this one had its moments.