It's obviously Orochimaru. He is both knowledgeable and intelligent at the same time. Let's take a look at his feats.
- Praised as a genious who appears once a few generations.
- Created tons of techniques (according to Databooks, creating techniques is his hobby).
- Achieved immortality.
- The greatest scientist of the series.
- Created his own dimension.
- Came up with a hypothesis on the Uchiha tablet despite not having Sharingan.
- Created perfect clones.
- Probably the most knowledgeable character of the series.
- Ran several hideouts with prisoners without getting caught, despite being number 1 in the Bingo Book.
- Managed to learn all 7 Nature Releases.
- Successfully disguised himself as Kazekage, fooling everyone during Chuunin exams.
What is your opinion? Do you think anyone could top Orochimaru in terms of intelligence and being smart?