As the title says, though Aizen is deadlier while fighting multiple opponents, but itachi one on one will **** up anyone with a mere glance, bleach characters like introducing themselves for some reason, that alone guarantees itachi's victory in his fights, Aizens kyouka suigetsu works in the same manner as genjutsu (controlling the 5 senses) bleach characters are weak as **** agaisnt genjutsu + kyouka suigetsu only creates illusions, genjutsu does more, paralyse/control opponents, create illusions, time manipulation, kyouka suigetsu does'nt do any of that, thus the solo king proceeds to solo another verse