nice thinking. And you are right, there is a BIG difference in having the strongest genjutsu technique (that is Shisui) and then being the greatest at using it. (that is Itachi) Itachi is the best at using sharingan genjustu.Once Shisui uses Kotoamatsukami he cant use in a decade(hundred years), so its pointless.
If he is known for it he must have used it right? lol it's between shisui and obito who did it to a perfect jinchuriki for years with out notice. itachi is elite and among the best based on him being a big role char. But as far as we know, it's between shisui and and tobi. I lean more towards obito based on the length and person he was able to do it to.Wow still? Come on its Itachi -_-
Shisui is known for his powerful genjutsu not his powerful use OF genjutsu.
Shisui is known for his powerful genjutsu, his not known for using genjutsu. Big difference.