Solo Leveling. I think after seeing so many recommendations (and better animes) I expected too much of it. It's just a very basic Isekai with flat, tropey characters and boring art style. It has some really cool fight scenes, I'll give it that. Forced myself to watch the first season to the end in the faint hope it'll do something interesting, but nope.

I guess it's watchable if you're looking for a simple power fantasy and don't mind tropes.

Season 3 of Death Note. I liked the bleak end of season 2, season 3 just felt like it came up with a contrived continuation to turn the tables. It was like a fanfiction.

Shield Hero. I loved the initial premise of an anti-hero doing what he could to just survive in a world that was against him. His only companion a slave that he was raising almost like a surrogate daughter, the two of them against the world.

I thought it was an interesting premise. Then the tone completely shifted and it turned into a generic power fantasy isekai where basically everyone loved him. Completely lost me the second they introduced Filo or whatever her name was.

For me it was My Hero Academia. I gave up on it at the start of Meta Liberation Arc.

Bleach falls off so hard after the Soul Society arc. The pacing slows down to a complete crawl and practically every subsequent arc just recycles the same tropes and patterns over and over with the occasional shocking moment or two.

Genesis of Aquarion had a strong concept but the execution increasingly fell flat for me as it went on.

Guilty Crown had a very strong start but the finale was so forgettable I fully forgot what happened in the last 3 episodes within about a week.

Mirai Nikki has a very strong start, a mediocre middle section, and a finish that would have been incredible if it made any sense with the show up until that point. The twist itself does work, but the emotional journey of the main characters is done in a way that's more edgy than it is relatable.

Darker Than Black is probably the biggest example of this. The dub helps fix some of the plot holes, but basically the showrunner who had the shows bible left the production about halfway thru season 1, so everything else after that point was the rest of the writers room just winging it, and it only kinda works, with the S1 finale being mediocre after such a great first few episodes. Season 2 is fully awful IMO.

Overlord. It started out interesting but then... nothing really happens. It's just a way for the author to showcase their fetish characters.

Wonder Egg Priority had so much potential but the last third of it was just so awful.

Schwarzes Marken, Neon Genesis Evangelion, Amanchu! Advance, Aldnoah;0, Horimiya: the missing pieces, Nyaruko, Lycoris Recoil, Magical Destroyers, VTuber Legend, the Misfit of deamon academy 2

Most of them started out strong but fell brutally off

aldnoah zero started pretty ok then dropped like a burning messerschmitt

Wolf's Reign.

Great soundtrack, terrible narrative.

I don't actually watch a lot of anime but reading this question, my immediate answer is PSYCHO-PASS. Season 1 is a masterpiece of a story regardless of the medium IMO. Philip K. Dick tier.

Then they carried on with it instead of stopping there. The movie was ok, season 2 was meh and the villain was a bit much. And I haven't even been able to bring myself to watch any other content from it. Not sure I'd want to because I prefer to pretend that sometimes it's about art and not always about money and that they stopped after telling the story they wanted to tell in season 1 without turning it into a franchise.

GAMERS.

a romantic comedy and a video game club? With real life copyright free references to games that we know..? With a catchy OP loaded with video game references??? I'm intrigued.

No let's use every fucking trope and make EVERY plot and conflict revolve around misunderstandings that could make be resolved instantly if the characters had more than 2 braincells and actually EXPLAINED instead of saying "ahhh eeehh ohh boy it's not what you think...!" "Why are you leaving? It's not what you think I swear!" look I can handle a few for plot sake but you can't make everything rely on the characters stupidity.

Also it was cancelled after s1 (probably for the best)

I really want to like Hellsing Ultimate. The same story I love, but turned up to 11! Nazis fighting the Vatican fighting Vampires, what's not to love?

Well it turns out when you turn everything up to 11, it mostly just becomes noise. A lot of the interesting subtle moments that I enjoyed in Hellsing were either dropped or abridged. Don't even get me started on the extreme tone flipflopping! I stopped watching about halfway through, and don't think I'll ever pick it back up.

Toaru Majutsu no Index has a lot of animes, seasons, spin-off and so on and when I tried to watch it I only got mad. Why a guy with amnesia can act like nothing happened? How nobody can see that your best friend/colleague/love interest/family is not the same person anymore? How your magic hand make you defeat trained soldiers with guns? Just shoot the protag for god sake.

Re:Creators I love the animation, the setting, the idea, the soundtrack but I don't know the show is so anticlimactic.

Biggest just because there was a lot of hype around it, but I didn't really like Jujutsu Kaisen as much as I hoped. Bad power scaling and weird jump cuts between scenes.

Spy x Family - has a weird vibe that is unsettling tbh, idk how to describe it and the plot seems slow and the relationships superficial

My Hero Academia - it's marvel superheroes (which I also don't like) and some of the powers people are born with just seem like horrific genetic conditions that bring nothing but misery for the individual.

spoiler

Like one dude has a spray bottle for a head. Are they even human anymore? Can this guy even eat food? IDK.

In recent memory: Zenshuu. I was incredibly let down by the isekai bait and switch.

Hoshi no Samidare. I loved the manga but the anime just isn't hitting. But perhaps that's only because of my expectations since I loved the manga.

I'll have to revisit that to see if it will hit after I have let it stew a while.

Deadman Wonderland is one of my top shows that I wish would get a remake

key the metal idol

Biggest one: Aldnoah Zero

Some potential contenders are: The Detective Is Already Dead, Akame ga Kill, and Shield Hero.

You youngsters.

Magic Knight Rayearth. Isekai magical girls with dnd fantasy elements and mecha. So, you know, little me's favorite stuff. The princess summoned them because she was too chicken to commit suicide and needed them to pull the trigger.

One piece, 100 episodes, and they tell me to continue, it'll get good they say.