Saturn was probably the most intelligent of the Gorosei, while currently it's Garling.
It makes sense that the former and current heads of science and defense would be the smartest among the Gorosei. People see Garling as the highly competent schemer, but Saturn seems to be overlooked. Saturn was a scientist who lived a long time and carried out many experiments. He had a lot of time to acquire knowledge. The conditions he set for Kuma were very thorough. He is the archetype character of an evil and ruthless genius, like Orochimaru, Bondrewd, etc.