Actually shikamaru is smarter, itachi smart is either a sharingan ability or his wiseness, which wouldn't be classified as smart but a form of manipulation, cold reading , and psychology.
Shikamaru. Being powerful doesn't make you better than everybody in every category. Not saying Itachi isn't smart. He is. Itachi is very wise. That doesn't equate to more intelligence.
Shikamaru doesn't have a Sharingan to read and predict people's moves nor can he place someone in genjutsu to ensure victory. He won his battles by thinking 40 steps ahead of his opponents.
Booksmarts go to shikamaru and overall smart. Itachi is just experienced, observant and calmly analyze situations. Itachi isn't "smart" like shikamaru.
I know, but because of the differing amount of exposure and character resolution Shikamaru and Itachi have, The former's IQ puts things into perspective. We can argue all day who's smarter here since Itachi and Shikamaru are their own smarts. But point out that Shika has 200 IQ and that puts him in a spotlight he definitely owns