1. "The Good, The Bad and The Lucky" (season 9, episode 6)
All The Flash had to do in its final season was produce a solid send-off for the Scarlet Speedster. It had 13 episodes to do it, which should have meant a more straightforward and coherent season, with one linear narrative helping it achieve that goal. And then season 9 actually happened.
Yes, we've spoken at length about the issues of season 9 before, but they really came to the forefront in this particular episode. "The Good, The Bad and The Lucky" was the annual waste-of-space that focused solely on the newer characters, as the team of Chester, Allegra, and Cecile had to figure out what was going on with Rebecca Sharpe (who was alive now after Crisis On Infinite Earths). Barry and Iris, meanwhile, were written out once again, much to the ire of fans.
In all honesty, this is actually a pretty fun episode. But it has absolutely no place in the final season. It could have taken place in any season, so it had no business being here. Equally, it could have been taken out of any season and nobody would have missed it. It's just there, existing to give Cecile, Allegra, and Chester a platform nobody asked it to, forcing the characters that the fans just didn't care for to the forefront of the show one more time.
This is the lowest-rated episode of The Flash on IMDb, and while it might not deserve that "honor", the show does for ever thinking that this episode was worthy of the final season.