Well as the secret leader of the organization and shadow mizukage of the water country, that would clearly be Tobi. He's got haxx mangekyo, hashirama's chakra combined with his chakra (wood style), six path power (Gedo power), yin yang release and skilled in his clan's Op Justus that are usually game changers.
I don't know any akatsuki member that can top that, he's got a little bit of everything into one.