Board Thread:Theories and Speculation/@comment-26545929-20150630030815/@comment-4061125-20150630040912

I'm gonna have to say Tsunade. She cannot be killed in battle due to her strength of a hundred seal which allows her to regenerate from any wound very quickly. Additionally, she has shown enough strength at full power to be able to shatter the ribcage of the Madara Uchiha's susano'o. She was also able to hold her own against him for a very long amount of time. She even punched one of his flame dragon jutsus to disperse it.

However, Orochimaru is a close second since he knows the impure world ressurection jutsu and is immortal. It's hard to say with him since we have never seen his strongest techniques because his hands were sealed for a long time and even when they weren't he was never in a fight where he was fully on the offense and trying to kill with everything he's got. We do know that he was a prodigy and he has exceptionally impressive defensive capabilities that go beyond normal ninja limits.