Itachi who knows and can perform the jutsu Izanami, said that Kotoamatsukami is the strongest genjutsu. But still, Izanami is the coolest jutsu we've seen so far!
No. He said it was Shisui's strongest Genjutsu - no the strongest in general.
Anyway, I wouldn't say that it's the strongest, either. Of course people think that it's the strongest right now, since it has just been released - that's the natural hype. And indeed, it's absoulutely amazing. It catches the enemy inside a never ending loop - definitely one of the most awesome effects a technique could have.
But Tsukuyomi, Izanagi, Koto Amatsukami, all those illusions are overpowered in their own way, too, and mostly can't be compared. Izanami one of the strongest, but can't be considered
the strongest. None of them can.