>>149356172
The reason Sivana isn't usually part of the conversation for the smartest guy is because DC doesn't use Sivana enough to demonstrate how smart he actually is.
Every so often you'll see him bend reality with an equation, bring back people from the death or reverse engineering magic, but since characters like Luthor, Batman, and Mr. Terrific are all constantly getting more use, they have more feats of intelligence for people to point at, and more reasons for DC to push as the smartest characters.