19. Great Saiyaman
Gohan's development in DBZ can best be described by the "upgrade, upgrade...NO! GO BACK!" meme. No character got more out of the Cell Games than Gohan, who as a teenager surpassed everything his father was at the time. Now that he's grown into a high schooler, fans were right to have the expectation that Gohan would take up the reigns as the show's true protagonist.
Instead, the writers saddled hims with a crappy comedy gimmick, made to parody heroes like Anpam Man and Kamen Rider. Gohan was steadily being built as a formidable fighter as he trained mainly in the background of his father's battles, but every bit of that slow burn was put immediately to waste when he took on such a silly appearance and exaggerated mannerisms.
If nothing else, the saga did introduce the new crop of Z fighters, like Goten, Present Trunks, and Videl. Unfortunately, these additions do not make up for the decision to ruin Gohan's character. After this arc went down, it was no wonder why the series spotlight again shifted back to Goku, leaving some fans beyond frustrated with DBZ's reluctance to let someone else take the mantle of top guy.
Few sagas in the series genuinely need a redo, but this one, if it had been done just a little different, could have dramatically changed DBZ for the better.