The power system of the Naruto world relies primarily on three main arts: Ninjutsu, Taijutsu, and Genjutsu. While ninjutsu involves the manipulation of the five basic natural elements, taijutsu is the art of hand-to-hand combat. Genjutsu, which is quite easily the most underrated of the three, focuses on illusionary techniques.
Genjutsu was initially introduced to us in Naruto chapter 6 on November 8, 1999, in Shueisha's Weekly Shonen Jump Issue #48. Since then, genjutsu have played a significant role in the story, even becoming the ultimate threat for the entire world of ninja at the end. Throughout the run of the Naruto series, there have been multiple powerful genjutsu users, but among the humans, none managed to reach the same enigmatic status as Itachi Uchiha, who was eventually surpassed by his brother by adulthood in Boruto.
Itachi Uchiha was a legendary user of genjutsu, and his control over these tremendously powerful arts was shown to us quite early in the story when he faced Kakashi Hatake in Chapter 142. When the two clashed, Itachi used Tsukuyomi, which allows him to create an illusionary world where he controls space and time. In the real world, Itachi had cast his genjutsu only for a mere second, but in the world of Tsukuyomi, Kakashi stated that an eternity had passed and that he had been tortured for quite a long time, breaking him completely mentally. Had Might Guy not saved Kakashi, things would have been even rougher for him. In fact, without the help of Tsunade, Kakashi would have been bedridden for a significant portion of his life, showcasing Itachi's immense capabilities when it comes to using genjutsu. Furthermore, Itachi's superiority when it comes to genjutsu usage was shown once again when he took on Orochimaru in a flashback. Without even looking at him, Itachi was able to completely incapacitate him in combat with a paralysis genjutsu.
“This is a special eye Jutsu. Genjutsu ‘Tsukuyomi’. Only another Sharingan user of the same bloodline as mentioned stands a chance to resist this.” — Itachi explains Tsukuyomi, Naruto Chapter #142
Even though there are multiple strong genjutsu that we have seen in the series, no ninja has had access to one as impressive as Tsukuyomi. Shisui Uchiha's Kotoamatsukami certainly comes close, but Itachi remains the only one capable of killing the target. What makes things even more interesting is the fact that Itachi himself had access to Kotoamatsukami through Shisui's remaining eye, which he had implanted in his crow for use. Itachi ended up using this ability in the 4th Great Ninja War, but had he been forced into combat, he could have also used this powerful skill, which further solidifies his status as the ultimate genjutsu user in the series. It goes without saying that, in the entirety of Naruto, there is hardly any character who can stand up to Itachi when it comes to genjutsu usage, even though many others went on to surpass him in terms of overall strength as ninjas.
“Calm down, I'm not being controlled by Genjutsu anymore. I layered a new Genjutsu on top of the enemy Jutsu to break free of the Edo Tensei, which has been cancelled.” — Itachi breaks Edo Tensei with Kotoamatsukami, Fourth Great Ninja War
Tsukuyomi made Itachi the god of Genjutsu in the first part of the series, but by the end, Sasuke Uchiha managed to surpass him in terms of the usage of illusionary arts. While Sasuke never gained a genjutsu as powerful as Tsukuyomi, he did have access to a Rinnegan Genjutsu, which was incredibly powerful. This was showcased to us following the awakening of Sasuke's Rinnegan in Chapter 673 on May 5, 2014. Given that the Rinnegan is a godly ability that is far stronger than the standard Sharingan, and both the Mangekyo and the Eternal Mangekyo Sharingan, it is no surprise to know that Sasuke's Rinnegan Genjutsu was on a completely different level.
Sasuke used his Rinnegan extensively in the battle against Madara and Kaguya, and the genjutsu ability that he gained was shown in Naruto Chapter 692. Using his genjutsu, he was able to paralyze and control all nine Tailed Beasts at once, which is a massive feat of strength. For context, the Mangekyo Sharingan only held the power to control the Nine Tails. With his Rinnegan, Sasuke was able to control all the Tailed Beasts at once and use their chakra, which was an immense feat.
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