Itachi *cheating* in Genjutsu?
Itachi is undisputably a genius and an almost completely unrivalled genjutsu master, but how much of that is due to Shisui's eye?
Shisui of the Shunshin was one of few Uchiha to have had the Mangekyou. His particular set of eyes was seemingly tailor-made for genjutsu. Among the numerous and powerful techniques he doubtless knew was Koto Amatsukami, a jutsu capable of convincing anyone that they truly believed something they previously didn't. This genjutsu is undetectable for the recipient, so if there are no witnesses it is a flawless brainwashing technique. Both of Shisui's eyes (left and right) posessed this ability.
Itachi's left eye can cast Tsukuyomi which (as you probably know) traps its target in an illusory world where he can freely torture his target (or talk to them or something but why would he do that?) causing enough mental trauma to put targets in a coma, if not break their minds completely. Sasuke also posesses this ability (in his right eye) though it is FAR inferior to Itachi's as he cannot manipulate time within it, and it might be easier to break.
Interestingly, the same eye Itachi uses to cast Tsukuyomi is the one he got from Shisui. Could this be a reason for his supremely powerful Tsukuyomi, even compared to his brother's? The eye Itachi used to cast Izanami was also his left. Is this a coincidence?
EDIT: Possible misconception in that Shisui's eye can only be used once every ten years: Koto Amatsukami can only be used in that time-period. The lesser functions of the eye can be used at all times, though whether it enhances genjutsu and/or Tsukuyomi is why I created the thread.