Smartest? I would say Tobirama from a couple different aspects of smartness. He is very innovative, inventing three jutsu that are used heavily. Minato never made a jutsu like Tobirama did, but rather took something he saw and made it smaller or improved on an already invented jutsu. Overall knowledge goes to Tobirama as well, no difficulty. Only Orochimaru and Madara are better in that area. I think they are tied analytically though.
If you mean like "genius" level (someone that does something new in their fields) then it be Tobirama. Inventor of Kage Bushin, Edo Tensie and FTG
If you mean by book smart as in who out of them knows the most about jutsus, then it would definitely be the Hiruzen. Given the title of the Professor. Able to discern obitos black charkra techs in a matter of seconds as well as deduce it wa made up of more than 4 elements as well as its weakness. Also countered 5 elemental jutsu with the same exact ones, able to tell each one right away. He also has his own jutsu he created such as the shadow shuriken (among others im sure).
Tobirama because without the FTG in the first place Minato wouldn't have anything and without shadow clones naruto wouldn't have anything. Hashirama is of course an idiot, And Hiruzen wasn't the one the built the leaf. Tsunade besides the medical core hasn't done enough to be the smartest.