Toji Fushiguro's actions in the Hidden Inventory arc shaped the entire storyline of Jujutsu Kaisen. Toji's overconfidence and failure to finish off Gojo led to crucial developments in the series ...
However, from a writing perspective, it is difficult to argue that Toji Fushiguro should hold the title of the best villain in the series since he reflects a lot of the main themes of the series.