It would have been the 4th Hokage but he's dead, sadly. Then I guess it's Naruto but he isn't as strong as he could be, yet. He's more strong of heart at the moment. Then there is all of the old Rookie 9. They are all really strong too, but not the strongest. So... at this moment it's either erk, what's-his-name. I know his name, it's... the one that has the DNA and jutsu as the 1st Hokage... or Gaara. Even though the Shukaku is gone, he's still powerful. Kakashi seems to slowly be weakening with the ongoing use of the Sharingan so he's not a canidate. And as for Jiraiya... he could be... but he has trouble against the Akatsuki so I'm guessing he can't be the strongest.