I can talk and talk, but ill try and make is quick. First thing that people confuse is intelligence and natural talent. Seems this thread is about genius i take it the op means intelligence therefore this is what ill say.
He was the only person in the whole of konoha which is made up of many various clans who sensed obito/madara and itachi isnt even a senser. Second is that he was able to monitor obito/madara whilst obito didnt know spot itachi. Third is that he was able to agree a deal with obito/masked man to help him kill the uchiha clan, but at the same time obito wouldnt touch konoha. This is a feat that a 12 year was able to accomplish.
Second is that during his introduction he was able to made a very quick tactic stragetgy and anaylsed kakashi, telling kisame that in a fight he would suffer unnecessary injuries. This later included the fact that he decided to flee rather than fight anbu, though some can same because his initial reason for konoha was for something else.
Third is the fact that he had planned his fight with sasuke. To push him whilst at the same time not kill him, that is a very difficult thing to do. In fact all of itachi's fights he has had to fight for other reasons rather than putting his opponents in a box and this ability is very rare and yet he has always achieved the desired outcome one way or another. Its easy to fight to kill because you are not restricted however fighting for other purposes always makes life difficult.
Then there is his fight against kabuto. Itachi had already analysed the situation even before kabuto started talking nonsense about him being closest to the sage, how he is immune to physical damage and how edo tensei ninja's are his pawns and started the loop of izanami in
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. It was later backed in chapter
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. He had already picked out a technique in his arsenal which would bring kabuto down but at the same time not kill him, from the very start of the fight. Very good quick thinking and summary of kabuto that itachi did, something that people seem to forget.
Itachi has shown to be able to analyse a situation very quickly and take action