Ok, so you have one little easy going guy named Shisui, right? He's your average Uchiha, with two scary red eyes, but that's all. He has a little dog named Bobby. Oh he loves that dog. One day he was walking Bobby through the forest. He let him off the leash so he could mark his territory. Long story short, a bear came about and killed Bobby before Shisui could react. Shisui got really sad and angry and awakened his MS. Embodied with this new power, he decided he would train, so no dog would die like that ever again.
Now follow me on this one, this is really important. So you have a guy with MS. Like every other OP Uchiha with MS, he was granted several techniques linked with each eye, cause having only one MS technique would be stupid and unlike any other MS user. Those techniques all happened to be genjutsu. I mean every single one. On a level of MS. And it just so happened that one of those techniques was a motherfu*king KA which was labeled as the most powerful genjutsu in history. So he had that one, along with other powerful genjutsu granted by MS. And it just so happens that he died long before story begun, so he didn't get any screen time, right? But that's why we have our old pal Bee to confirm Shisui being the best at genjutsu. And Itachi not disapproving that. So there.
And of course people, if you are gonna label Itachi as the best because we haven't seen enough of Shisui, at least stop being hypocrites, cause I see those same people on other threads claiming that "prime Hiruzen" belongs to top ten. But we haven't seen that in the manga, did we?
Ok, good, I'm off.