This is not another wank thread but just something off premonition.
So to get back on topic, we all know that Shishui's eyes is said to be the most powerful genjutsu (besides IT) in the whole series with virtually know way to counter it.
Itachi and Shishui were both very powerful and wise, but does anyone still think that Itachi is still stronger than him if they were at the same age? I know I do.You must be registered for see images
I don't think they ever compared each other in terms of power in the manga.
Itachi just have this vibe that he's stronger than Shishui but it may not be the case. And if they all went out to show their complete susanoo's it would've been a different level of respect.
If Shisui's eyes are not on cool-down (yes, this limitation applied to him as well- otherwise Itachi would have no way of knowing that his friend's eyes were dormant for exactly a decade between use unless Shisui told him), then he likely would win with KA because that captures anyone in Shisui's line of sight.
^Then again, this might not work as when Itachi was expecting the crow as he met Naruto, he was aware that the jutsu had taken place. Itachi knows of KA and would expect it if Shisui were in the position to use it. We also know this genjutsu is the most potent in terms of controlling someone, but could someone stubborn enough override the request? Itachi is very stubborn when it comes to his goals/intents. Or, can the victim just shallowly follow the request? When Itachi was under "protect Konoha", he didn't go back to the village to protect it directly, he went to go stop Edo Tensei like he wanted which would indirectly benefit all the villages. If they were fighting and Shisui gave the most useful command to win a fight (ie: kill yourself), would Itachi kill himself right away or would he be able to defeat Shisui first and then have to eliminate himself? We don't know enough about KA yet but from what we've seen this is not a deciding factor by feats- only speculation.
If Itachi and Shisui were the same age, I would say Itachi would be stronger simply because he was more skilled (with what he did do at his young age) and progressed quicker.
But Shisui is really strong as well and should be the strongest Uchiha after Sasuke/Obito/Madara/Itachi.