In Bleach Aizen seems to be the strongest villain so far. However I doubt his illusion powers would work on Itachi because of Itachi’s sharingan. Illusions are Itachi’s specialty. I think he’d turn the tables and trap Aizen in a genjutsu. Itachi wins.
Updated. Aizen got a new super form and now his powers surpass that of Itachi’s. Aizen wins.
DR
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Aizen would have won even without the transformations.
Hmmmmmmm, It’s definitely close, but yeah he would
Still useless against Itachi’s Tsukuyomi and besides, it Aizens new form wouldn’t even help against Itachi’s Susanoo! Itachi wins.
With Aizen’s new super forms he’s attained a power not seen since DBZ days. Itachi couldn’t take him down. Itachi’s a strong warrior, but few are Aizen level