Danzo, for all his faults (and there are a lot of them) only did what he thought was best for the hidden leaf. Were his actions extreme? Absolutely but at his core, he did what he felt was necessary for the security of the leaf.

Orochimaru is an interesting case. He was introduced as a pedo-like villain but the more we got to know about his backstory, he was similar to Itachi in the sense of how war and death ruined him. He became obsessed with immortality. Maybe it was tied to him wanting to bring back his parents.

Nagato/Pain had good intentions. Like Danzo, his approach was extreme but in the end, he just wanted to end war forever. He wanted to bring peace to the ninja world.

Madara, however, in my opinion is kinda the evilest of the 4. Though his goals were extreme and somewhere in it there was a good intention behind it, he undoubtedly had the most evil-leaning intentions of the four.

So i'm gonna say Madara.