One of the worst from me was Fused Zamasu becoming Infinite Zamasu, it was completely needless, his appearance is literally a poorly done jpeg in the background and it extended the length of an arc that was already too long and led to one of if not the worst arc resolutions in DB, with them just using Zeno as a ctrl+alt+delete for their problems and Trunks’ timeline. A runner up would be all of Natsu’s nakama power boosts and ability to power amp himself up with other elements even though his whole thing is fire. Meliodas is another one, I’m not sure if that counts though because every time he gets a power boost it’s not because he gets stronger but because “he’s unlocking more of true power from his prime demon days” or when Nakaba retconned his loss to Escanor as “him not going all out and letting his guard down” even though the dude practically flexed him into near death coma. Naruto and Sasuke’s SO6P power ups where they jump from kage level characters to untouchable...