Obito... There was a lot of potential, but I honestly did not expect Kishi to rewrite his character that much (Tobi died when the mask broke)... neither did I expect the backstory to be so 'empty'. Especially after all the sinister and tragic stuff Kishi put Gaara, Nagato and Itachi through.
And Izuna came second, mostly because two brothers working together on this masterplan can be pretty epic.
And Izuna came second, mostly because two brothers working together on this masterplan can be pretty epic.