I'm not sure but these are the possibilaties (in my opinion)
Itachi: Wiped out his entire clan... without a scratch!
Kisame: I got nothing here
Deidara: Got a Jinchuuruuchi (or whatever they are)
Sasori: Didn't he survive a horible accident? Is that why he has puppet parts?
Orochimaru: We all know he is extremely strong.
Tobi: Claims to have killed a demon.
Zetsu: He is a plant. You have to have a really freaking cool power to be a plant.
Akatsuki learder: Obviously, he is the strongest.