Obviously Madara. For once, someone important wasn't interested in Naruto's "let's all be friends" ******** . Only he of all characters had any chance of filling the 'final villain' role without turning the plot into even more of a trainwreck than it already was. He was so close to pulling it off.
Then all that Zetsu/Kaguya nonsense happened. The train didn't just crash, it imploded.