Aizen's by far, Genjutsu tires you out, Genjutsu can be broken, you need to make eye contact every match and it can only be used on people with Chakra, Kyōka Suigetsu doesn't have any of these lame restrictions =D
if aizen had that soul sphere thing I bet Izanami destroys him 100%
if he doesn't, then he's still screwed.
either way aizen loses.
the thing he used to morph that also granted him a huge power boost
Aizen, his shikai is super overpowered and his bankai has yet to be revealed. Itachi has well.... finger genjutsu... thing is well... isn't Aizen immortal too?
we all know aizen is better because his is similar to kotoamatsukami in the way that it's almost a perfect illusion. itachi's thus far have been obvious to the victim that something was very wrong. so IMO i would pick aizens for the overal tactical advantage i believe it has. aizen is faster than itachi can ever dream to be and obviously has more power in 1 vs 1 fight. IMO itachi stands no chance in hell.
it doesn't matter if they know it's an illusion or not. and they obviously know it's an illusion if itachi is torturing them on a stake.
the problem is how would aizen escape it?
aizen: sword snap me out pls
sword: how??