9. "Mother" (season 7, episode 3)
The Flash season 6 was such a comeback for the show. Its first half was admittedly better than its second, but there was still a lot of potential there in the Mirror Monarch storyline, which saw Efrat Dor deliver a chilling performance as the villain. Unfortunately, the pandemic resulted in the season being cut short, meaning that it needed to be wrapped up quickly in the first few episodes of season 7.
That conclusion came in "Mother", where Team Flash took on Mirror Monarch and all of her mirror children that were destroying Central City. It's unclear if this was still the original plan for the season 6 finale or if it needed to be reworked now that it was just another episode of season 7; either way, it came off as very rushed and incredibly underwhelming.
The fact that Eva McCulloch was set up to be such a formidable threat only for her to be defeated by a pep talk left fans feeling a little cold, while the constant reminder that Barry Allen couldn't recognize his own wife had been taken into the Mirrorverse also didn't sit well with them.
Look, it was serviceable when you consider everything they had to account for during the pandemic, but it did feel a bit disappointing considering how solid the storyline was leading up to.