Zetsu : The Akatsuki was a group of powerful S-ranked rogue ninja who worked as mercenaries, but their true goal was to capture the Tailed Beasts in order to change the world. Aside from Zetsu, every single member of the group was a Kage-level ninja, which means that they were both strong and smart, but when it comes to pure intelligence, some members were smarter than others.
Black Zetsu Manipulated Events For Centuries
Zetsus was technically two beings in one body, and White Zetsu was more or less an Infinite Tsukuyomi victim who became a mutant who did whatever he was told. Black Zetsu is a completely different matter though, as he was actually the physical manifestation of Kaguya otsutsuki’s will.
Zetsu is by far the smartest Akatsuki member, and that is because he managed to manipulate events for hundreds of years, to the point that he was technically responsible for most of the ninja world’s longstanding conflicts.