With prep, Kabuto wins for sure. Kabuto is probably the most dangerous character in Naruto if he has prep. He can create the Edo army and has other hax techniques besides. Although Nagato would win with no prep involved.
Yeah Kabuto is strong even without Edo. Pre sage made and bringer of life jutsu as well as Orochimaru's powers, he could hold his own against Tsunade. He is far more powerful with the forementioned abilities. Of course i still see Pain winning with no prep involved but not quickly at all.