Nagato to me had the most potential in the whole of naruto, we didn't even get to see half the things he could do.
But then again the manga isn't called Nagato. Things would of gone different if he was in control but let's not take anything from itachi, he is another good character who can be...
Nobody has seen minato do s genjutsu so that would be like saying naruto.
Id say technique wise shisui but I think it call comes down to how you use it so id say itachi because he is a genius and we have seen him use it many times...
But Madara genjustu is suppose to be on another level, I'm...
Maybe the so6p gave each of the beasts what was left of his power or something important part each and when the beast found someone that resembles the so6p. Also where the hell is the first son!!!!!! I seriously think Tobi is the first son. That would explain his knowledge and longtivity.
I...
I Think 'That' justu is the powerfullest sealing justu and the reason minato left it for naruto to complete is because non jinchūriki cannot do it.
Great theory though