I agree, but it should really be Madara, Obito has only been in since chapter 600, all before he was known as Madara.
Regarless of that it was still obito and i agree with the OP because obito was the one directly responsible for most of the tragedies and defining moments of the main characters:
1.Obito was the one orchestrating the nine tail attack which killed minato and kushina and made naruto an orphan and jinchiriki and made the leaf suspicious of the uchiha affecting sasuke and itachi.
2.Obito was directly involved in the uchiha massacre.
3.Obito was the reason itachi joined akatsuki.
4.Obito turn the original akatsuki and nagato which were working towards peace into an evil organization.
5.Obito was the true leader of akatsuki making obito responsible of their actions including garaa´s death and the destruction of the leaf.
6.Obito was the one who told sasuke the truth of itachi.
7.Obito was the one who recruited sasuke to capture bee and sent him to the kage summit, making sasuke an international criminal.
8.Obito was the one who declared war on all 5 great nations causing the death of more than 40000 shinobi.
9.Obito killed konan and neji.
10.Obito gave the sharingan to kakashi that made kakashi who he is right now.
11.Obito gave kakashi the moto "those who abandond their friends are worse than trash" that ultimately was passed on to naruto.
And many many more, Obito is indeed the one who is responsible for the most defining moments of the life of the main and major characters like naruto, sasuke, kakashi, nagato, minato etc, As a matter of fact there´s not a person whom obito didnt affected in the whole narutoverse.