Itachi's genjutsu is superior, including skill wise between Itachi and the second Mizukage. The Sharingan specializes in genjutsu and unless the opponent of the sharingan user is skilled in chakra control like Minato, Jiraiya, Tsunade, etc, it's best not to face the sharingan 1 on 1. Tsukuyomi has been stated to be the most powerful genjutsu in that its weakness is that it can only be cancelled by someone who is related to the Tsukuyomi user by blood. Skill, of course, plays a huge factor in the genjutsu as well, as seen by Sasuke vs Danzo. Sasuke couldn't use Tsukuyomi to the level of Itachi.
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