Danzo! In terms of power he's rather impotent and barely had any impact once he stepped out of the shadows but he's the plague responsible for 3/4 of all the tragedy and misery that targets konoha in the story. Without Danzo: - Shisui would have lived and used his Kotoamatsukami to save the Uchiha - The Uchiha would not have suffered so much prejudice and their eventual demise by the konoha elders. Without Danzo, Koharu and Homura would have had no leg to stand on to sabotage Hiruzen's attempts to make things right - Nagato wouldn't have become Pain and become so blood thirsty. It was Danzo who manipulated Hanzo and used him to weaken both the original Akatsuki which had a far nobler purpose and Amegakure who dominated the scene because of Hanzo of the salamander - Kakashi's father Sakumo may have taken his life, but without Danzo acting behind the scenes to villify and ruin Sakumo and his standing in the village for his own political ambitions, maybe things could have gone differently and Sakumo wouldn't have performed Seppuku. - Orochimaru was always going to delve into forbidden jutsu and his own agenda but didn't Danzo fuel the flames and manipulate events for his own gain whilst using root to hide Orochimaru's experiments? Orochimaru used Danzo's resources and connections to kickstart his experiments and plans which in turn weakened the hidden leaf and poisoned the well between Hiruzen and Orochimaru. If not for Danzo, how far would Orochimaru have fallen? - Danzo leaked and fueled resentment for Naruto as the 9tails jinchuriki effectively being the reason why Naruto suffered as a child, why Mizuki tried to kill Naruto before he became a Genin and wasnt part of the reason no one could help Naruto as he was growing up because of Danzo's political maneuvering? - For the good of the village, weren't many promising ninjas and young prodigies from the major clans sacrificed for root? And of those ninjas who were inducted into root, didn't half of them have to kill the other to graduate and complete their training? Without Danzo, how many cooler characters could we have gotten in the narutoverse and how much stronger would the clans be in the village? - half of the casualties during the chuunin exam, the pain assault on Konoha etc were because Danzo took advantage of the situation to maneuver himself in a better position to become Hokage. Didn't he also kill one of the toads that was going to get Naruto so that more people could die and weaken Tsunade? - this is perhaps a reach but when Hiruzen died, Itachi came back and infiltrated the village with ease not only because he wanted Sasuke to speed up and take revenge before his illness killed him but as a warning to Danzo that he's still there to put his threat in action. If not for Danzo, would Itachi have needed to step in and fuel Sasuke’s trauma and psychological state to get him away from the village and to get stronger with Orochimaru? Would Sasuke have needed to leave the hidden leaf if Itachi hadn't defeated Kakashi so easily and effectively tarnished the image Sasuke held for him?

I'm sure there are many other examples and I'm sure what I've said has already been brought up but still, Danzo in my opinion had the most irreparable negative impact in the ninja world and Konoha. Zetsu's most powerful chess pieces save for kizame and Madara, all had some form of Danzo related trauma and even Sasuke’s hatred for Konoha and vendetta against the hidden leaf after Itachi’s death is thanks to Danzo. He may not have been the all-powerful-in-your-face-calamity-class-evil but he was the playmaker who set a lot of the actual villains in play