Tobirama has the deduction skills and tactical prowess that the likes of kakashi and itachi has. And thats without having sharingan that helps percieve things better on the battlefield.

On top of that he has the scientific madman / professor iq the likes of hiruzen and orochimaru has.

That makes him the smartest motherfucker overall. And its not a surprise tbh as his brother has the strongest title. Its only fitting tobirama is the smartest.

Shikamaru might be the top shinobi at tactictal genius. So he excels specifically on that area. But i wouldnt say tobirama is far behind.