1 – Clover
The greatest scholar of the Oharans. With his guidance, the scholars of Ohara were able to figure out the existence of the Ancient Kingdom, their war with the World Government, and likely many other mysteries of the Void Century and the years around it. Archaeologically speaking, Clover is the smartest man in One Piece (with exception to Nico Robin at the end of the story most likely).
2 – Sengoku/Tsuru
The great mastermind strategists behind the Marineford War. Without them calling the shots from behind the front line, that war may have ended very differently.
3 – Benn Beckman
Benn Beckman is stated to have the highest IQ of anyone in the East Blue, which ranks him above Kuro and Nami. We haven’t seen him much so we don’t know much about just how smart he really is, but I think it’s safe to say he may be the strategist of the Red Hair Pirates, playing a Nami role of sorts.
4 – Sir Crocodile
Crocodile is, to say the least, extremely savvy. He knows what the hell he’s doing. He’s extremely knowledgeable of the One Piece world due to the fact that as a rookie he made it far enough into the Grand Line to fight Whitebeard himself, and although he lost that fight and retreated back into Paradise, he certainly learned a lot from that journey and from his time as a Shichibukai. He was smart enough to assemble his own organization of assassins and bounty hunters in a scheme to take down an entire country and he knew about the Ancient Weapon Pluton and the fact that it’s located on the island of Alabasta.
5 – Nico Robin
Archaeologically speaking, Robin is likely the second smartest character in One Piece, only behind Clover. She’s discovered many Poneglyphs along her journey and likely knows pieces of history that are very uncommon knowledge. Putting her above Kuro may be a stretch to some, but while Kuro certainly has her beat on the whole scheming part, Robin has him beat in general knowledge of the One Piece world and its history.
*List by Pirate-King-Ace