To be precise, the smartest man in One Piece at the time the question was asked in SBS (around Arlong Park Arc if i'm not wrong)
That's why I carefully write one of the smartest person in One Piece (not the smartest person in One Piece) since I am fully aware that could be another person that is smarter than him in OP.
Nonetheless, he's still one of the smartest person in OP as what I've carefully mentioned above and it is pretty interesting since none or at least not many potential high tier level or above characters that was specifically and explicitly mentioned for their intelligence.