My vote goes out to the Orange Hokage,
I'll obviously win this discussion seeing that the story is all about him

and he's already surpassing Jiraya and Minato (according to the old sage) at the age of 16 ;O adding that he does happend to have the strongest of all the tailed beasts (the 9 tails) inside him.
And according to the 499-500 episode he clearly managed to control it (we're just waiting for him to start kicking ass using both the 9 tail chakra to it's max mixed with sage mode!.
To avoid being a cheat and choosing the main char of the series.
I'd probably go for.....
The first. sadly I just don't know enough about them, but the fact that the 1st has defeated Madara at his prime just comes to show how strong he is.
I know people will argue that Minato could have done the same, but there's no way to prove it. We don't even know if that masked guy really is Madara, and even if he was, he did tell all the kage's that after the fight with the first, he isn't even a fraction of what he was. So how strong was Madara back then?
But I wouldn't change my mind about the 5th, she's an awesome Hokage. She may not be as strong as the others, but her healing abilities far surpasses any of the others and with that she allows her ninjas to have a fighting chance against other villages.