Genjutsu is one of the three main types of Jutsu. It falls under the category of Yin Release, and affects the cerebral nervous system, which allows a person to freely control the sense of the victim. Itachi Uchiha was one of the strongest Genjutsu users in Naruto, but there are a couple who exceeded even him, in terms of prowess.
I’ve listed all 5 below—
5. Shisui Uchiha
Shisui Uchiha was the strongest known Uchiha of his generation. He was also known for his speed, and had the moniker, “Shisui, the teleporter.” He had also awakened the Mangekyo Sharingan during the 3rd Great Ninja War. This allowed him to cast a Genjutsu called Kotoamatsukami. This Genjutsu is said to be of the highest order, and allows a person to subtly control others. After his death, Danzo used this Genjutsu but his skill was nowhere near Shisui himself.