I guess mine'd be black zetsu, he's a rather nasty one. He screwed up all the warnings Hagoromo left behind, turned basically the entire history of shinobi and origin of chakra into a lie. Ensured sibling rivalry continued instead of ending with Madara and Hashirama, only added fuel to the flames. Had a hand in orchestrating Danzo getting Hanzo to betray the original Akatsuki to recruit Nagato. Fooled Madara and prolly even all white zetsus into thinking he was just a pawn Madara created, and didn't just dick over Obito but fought Edo Minato and Kakashi in his half dead body, dicked over Madara as the juubi host even worse.
I despise Danzo because there seriously is nothing to like about the geezer, not one bit. For some reason though, black zetsu was significantly worse, but can't hate him.