shisu is not the strongest itachi defeated him aand sasuke surpassed itachi in term of fighting style and ninjustu. We dont lot about madara though.
Seriously, Sasuke surpassed Itachi? When? When Itachi let him win, could've killed or sealed him any time, etc. If not for Itachi, Sasuke wouldnt have achieved the MS. The entire fight was orchestrated for this and to purge Orochimaru from him.
Sasuke may have a better Ameratsu with his Enton, but Itachi's Tsukiyomi is far superior as he could control space
and time. Itachi's Susanoo is clearly superior due the spiritual items. Sasuke has the EMS, we'll see what that does to improve Sasuke's doujutsu. However, Itachi always fought with a sharp mind, while Sasuke has succumbed to his emotions. If he corrects that, his fighting style would increase dramatically. Sasuke probably will surpass Itachi for the story, but I haven't seen it yet.
i think you are overrating what he could have become. shisui was related to itachi, but it was itachi who fugaku treated like the future clan leader, not shisui.
yes, mind control is nice, but did you ever ask, why danzo couldnt use it for an extended period of time? it has a time limit and a cooldown, and the cooldown is obviously endless.
how strong his mangekyou would have been doesnt play a role for the question who the strongest was/is, because he didnt get it.
if you ask who the strongest uchiha was, i have to say madara, who was the only one who could use ems so far. i doubt that shisui could beat madara at his prime, simply because ems outclasses three-tomoe.
I agree Itachi is superior; however, his father also had a paternal bias for his sons, though it seems like more towards Sasuke because of Itachi's recent behavior at the time. Shisui's strength with a MS I'd speculate to be around Sasuke's, maybe weaker but with a more tactical use of the MS doujutsu, as spamming MS doujutsu isn't tactical fighting.
Right now (subject to change once I see Sasuke's EMS.)
Madara-Itachi-Sasuke/Izuna,...,Shisui and other elite 3-tomoe Uchiha.