Why do people refuse to pay attention to the topic.What about Madara's IT? If we include it, then Madara is NO DOUBT number one. Followed by Shisui's KA and Itachi's Tsukuyomi.
Its not about the power of genjutsu, but about how skilled someone is in genjutsu.
Itachi's whole fighting style is based around it, he can cast genjutsu from clones, crows, fingers, eyes, he can even genjutsu people without the need of 5 senses, even people who shut off their vision and are not supposed to fall under genjutsu.
He is most skilled user in genjutsu.
IT, Koto > Tsukuyomi
But feats show us he was most skilled in fighting with genjutsu and incorporating genjutsu frequently in his tactics/battles.
Its about the users' personal ability and skill, not just the power of a jutsu.
And so far, Tsukuyomi i'd say its the most useful in a battle since you literally cripple your enemy, while Koto can simply make one attack his teammate but can easily comeback to normal by having someone disturbing the chakra, while Tsukuyomi damages the enemy in a strong way and even if somebody disturbs their teammate's chakra, the damage is done.
IT has only shown us to put people in a fairy dream, with no dmg whats so ever.