Thank you! I appreciate that.
I'm sure it must have sucked for the animators and writers going from Protect Chibiusa! Clash of the Ten Warriors to that.... You go from the Senshi finally having a showdown with the Four Sisters to.... a beach episode where Chibiusa makes friends with a badly drawn Lapras? I feel for them.
Technically, the beach episode was before the showdown.
I don't feel bad for them at least on that front because animated series don't work like that. Typically, multiple episodes are animated at once in batches; there is no way to reliably release an animated episode a week with a serial schedule even if the animators are working at breakneck speed and you employ all the cheats in the world. Basically, the animation team that worked on the previous episode was likely not the same team that worked on this episode, and the animators don't get to choose what to animate.
In fact, the script writer of that episode
actually wrote a lot of pivotable and memorable episodes, which makes this misfire even more aberrant. I speculated someone in charge probably mandated a beach episode and due to the time crunch, the writer had to come up with something and this was the result.
Plus, with the anime trying to somewhat be parallel to the manga, it needed more padding. It has to be hard to develop a series when the actual material is ongoing and it doesn't seem as though Takeuchi had a long-term plan for the story, at least from the writing. That makes it hard to develop any sort or meaningful story and makes the show reliant on "filler."
As far as the original question, another hated episode for me is Jupiter and Venus' R power-up episode. I hate it because it is surprisingly bland for a power-up episode and was dependent upon contrived drama between Makoto and Minako. Even in the old dub, its title, "A Charmed Life," is just a recycled name of the debut of Sailor Mars "An Uncharmed Life." No, there's no intentional mirroring. It's just reused.