most of what i wrote below is based on hype if you base things on the manga then it is jiraya probably or tsunade but mostly jiraya (guess m kinda of an oro fanboy huh.... didn't know that till just now

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based on what we've seen jiraya or tsunade but based on what i think honestly it has to be oro we've barley seen any of his jutsu so i think its not fair o oro. in part 1 it said that he was supposed to be hokage i mean hiruzen probably knew tsunade or jiraya's potential why not choose them? i just think oro is really underrated and that he's more powerfull than people think
also i have to say oro vs tsunade was not fair tsunaade got over her phobia oro couldn't exactly make his hand work on sheer willpower could he? not to mention his arsenal heavily relies on ninjutsu which he couldn't do without his hands tsuande was fighting a handicaped and dying orochimaru that isn't what you would call a fight is it? also we all know if jiraya wasn't poisoned he would have just used ssage mode to kick oro's ass.
oro fought jiraya and im pretty sure jiraya had sage mode infact jiraya himself admitted oro was stronger than him. so i just can't see tsunade beating a healthy oro who was praised by the third for his skill and known as a genius in the leaf. but i can potentially see jiraya taking him on as an equal. i have other reaons too such are the main ones so my vote goes to oro.
tsunade if you ask me is by far the weakest sanin IMO (no offense to tsunade fanboys so please don't attack me because all my claims are based on hype anyway)