Probably a tie between
Kidou Senshi Zeta Gundam and
Re:Creators
For the former: So bad that it made me "turn off my brain" the same time you computer blue-screens to avoid further damage.
Pacing is atrocious, anime is repetitive (I kid you not that at some point there are four episodes in a row where the protagonist is taking a beating the entire episode, finally lands a hit, then the enemy flees). There is only a single episode I managed to like (Episode 39, as I felt that it "harvested" everything that it had "sowed" so far).
Plot happens off-screen, leaving us with only pointless fights. If a fight is a dialogue, then Zeta's fights are two characters spouting gibberish, as there's never a personal reason for them (Even the protagonist's adversary, at some point, admits that he isn't fighting out of vengeance or anything like that... then why is he fighting?".
Villains are mustache-twirling ones and no one ever explains their motivation towards having their own positions.
For the latter:
One episode in we're told "In X days there will be a fight against me"... at the end of the anime, there is that fight foretold in episode one, which begs the question "What the hell was done in the meantime?".
Nothing, the answer is nothing. The non-diegetic explanation was that the antagonists design made her hard to animate, so she does nothing for 90% of the anime.
Each character is supposed to represent a character of a certain genre, put after their first appearance they all become generic and without a personality, losing the point of the premise. The anime waster a lot of time doing nothing (The anime could 100% be written without it's protagonist, whose only action is demanding personal validation and omitting relevant information from everyone else).
I could talk about it all day long, but let's just use two examples:
- There's a character who gets their power stolen... this never becomes relevant, as he never fights or needs to use that power after that, and in the single moment he needs his power back, he gets it back immediately.
- The character of Magane, whose only purpose is being a "plot device" at the end (Despite the fact that we already have Meteora as a plot-device character... ah, and Meteora also acts like a constant infodump) doesn't even get her story concluded nor appears in the epilogue.