Re: Itachi boy genius.
There are different kinds of intelligence. Kakashi was a chuunin at 6 and jounin at 9 or 10 because of his tactical intelligence(amongst other things, but this is clearly one of, if not the biggest reasons). His tactical intelligence feats in-battle are not matched by anyone in the manga.
However, that isn't necessarily "hokage type" wisdom/reasoning, as that only pertains to the battlefield.
Orochimaru, as another example, is not matched in knowledge of jutsu. That's not "hokage type" wisdom/reasoning, as it only pertains to techniques.
Claiming there is a single "smartest" shinobi in the Narutoverse isn't the right way to go about it. You have to break it down. Think of it as similar to the difference between being "street smart" and "book smart" in real life.