So for me it would have to be the original Dragon Ball tv show. I fell in love with the show around the Tien Saga for how meticulously animated the fighting was. Every punch and kick was seen and the focus was on martial arts. This continued through the King Piccolo saga, which I also loved.

However, I was massively disappointed with the Piccolo Jr saga. To me, it seems like the show switched from being focused on martial arts to generic energy blasts and poorly animated “flurry of blows” that really watered down the quality of the fights. Where before we had well drawn punches and kicks now we had people yelling loudly while blasting energy beams.

I’ll die on the hill that this was a “jumping the shark” for the Dragon Ball franchise that it never fully recovered from. Future DBZ seasons were a bit more creative (thinking of Goku holding Raditz while Piccolo blasted him) but the show as whole never really returned to being about “martial arts” the way it was before.

I feel like an old man shaking my fist at a cloud though, as I’ve yet to meet one single person who misses that focus/aspect of the old show.

Promised Neverland, though I only watched the first season due to how much they changed for the worse in season 2. I've read most of the manga so I knew what was supposed to happen, but I've never got around to finishing it.

I don't watch a lot of anime, but how about the original Evangellion? My understanding is that the last few episodes were deeply confusing and disliked when they came out, to the point that the movies were created partly to "fix" the ending.

Controversial, but Attack on Titan.

When they realised how much money they could squeeze out of the IP, they decided to stretch the story way to long. It should have ended already.

Surprised nobody mentioned Tokyo Ghoul, it's probably one of the most legendary fuckups in anime history.

Also the second season of The Promised Neverland.

Since the worst examples I can think of have already been mentioned, I'll just throw in that Boku No Hero Academia is making me pretty sad these days. It's like they decided to stretch out a few things that were shorter in the manga for a reason and it's gone on for so long that it doesn't feel like the same story at all anymore. I miss what the show used to be.

ETA: Ascendance of a Bookworm was really fun right up until she decides to dump everything in the trash and join the church.

This isn't like awful, but it's noticeable. Full Metal Alchemist (brotherhood? I forget who came first) we started great and strong, good pacing, but then they ran out of source material too quickly. SO, they started making it up as they went. I stopped when the general planning a coupe flips to a genocidal maniac, the character shift was so jarring. Apparently further down this path they lose their alchemist abilities and get shot to WWII to fight some Nazis. It was so off the rails, when the manga started up again they completely redid the anime from the beginning. I love this anime, but I have to make a guide on how to watch it every time I suggest it. The second attempt didn't do as well with the early episodes like the first one did, probably because they were trying to catch up. Wild ride, still worth it.