Hum not sure why people say Shisuis KA. If we talk in terms of power, it's toad genjutsu the strongest, since it can kill multiple targets at once, something that none other genjutsus has shown to be able to do
Shishui's KA makes the victim do whatever he wants... He can easily use that on someone that can kill multiple people, it completely changes someone's live. Just as if it would've ended up being used on Sasuke, if sasuke does get to kill anyone from the leaf, if itachi's "protect the Leaf" would've affected Sasuke, Shishui's KA could have also saved lives.