Tamara

neglected to include the key - Itachi's mastery of genjutsu as applied in the tsukuyomi is likely superior to all Uchiha, with the exception of Madara. It's not a unique ability, in fact, it's arguably the basic power unlocked by the Mangekyo (rooted in the initial developers/wielders of chakra, the Otsusuki). It's strength depends largely on who wields it (nod to Black Zetsu and Kakashi). When comparing different jutsu used by the Uchiha, or anyone in the Naruto series' highly noted for their skill, it's all about mastering a technique - can you do it, and how well? Genjutsu is particularly difficult to perform or break out of when used by an intermediate-level user, and sucks up the caster's chakra to a great extent. Having a Mangekyo ups the ante...employed by users with incredible stamina, vast chakra reserves and skill. They call it visual prowess for a reason, right? Though I can understand that it can be unclear what each Sharingan type delivers a "level-up" ability to those who achieve it, versus an evolution of an innate/unique user ability, AND just how complicated it is to compare apples to oranges even if they are in the same basket.

Itachi initially awakened his Mangekyo when Shisui died...which was shown multiple times in Shippuden from different perspectives and once (maybe, twice?) In Naruto around the first appearance of the Akatsuki/Sasuke's defection from the Leaf. Shisui had the most formidable genjutsu in the series (aside from the infinite tsukuyomi) evolve upon his Mangekyo awakening. Not sure that expressions of each MS user's natural abilities were taken into account here, or if the complexity of the various Sharingan levels/powers shown and explained in depth by various characters throughout the majority of Shippuden was simply overlooked.

Remember, Itachi planned to give Naruto Shisui's eye, not his own (though his were failing by then), for that reason...plus, Itachi had to go to great lengths to employ Shisui's genjutsu against Kabuto, noting how superior the ability was while explaining it to Sasuke during their pursuit/battle with Kabuto during the 4th GNW.