It would be senseless to assume this last jinchuriki isnt the most powerful. One, why wouldnt Kishi save the most powerful for last? Meaning it would be pretty stupid if Kishi saves this jinchuriki/bijuu for last only for it to be weak and easily beatable. What kind of suspense would be built for Sasuke and Hawk's ensuing fight with the 8-tails if he wasnt the most powerful? And why would Akatsuki target the 8-tails last (besides Naruto) if he wasnt the most powerful? Obviously those of us saying he is the most powerful are not basing it on the few panels he's been in.