Board Thread:General Discussion/@comment-24382631-20140106203344/@comment-8986886-20140107052959

Since Aizen eventually became so powerful that no one save Ichigo could even sense his Reiatsu, I'll use Ichigo and Ulquiorra as measuring tools to try to reason out Aizen's strength. By Ichigo's and Unohana's conversation in ep.288 (@16min), Ichigo’s Bankai had around twice the Reiatsu compared to the average captain’s, at the time he fought Aizen in Fake Karakura Town.

When Ichigo dons his Hollow mask his power reaches yet another level, commented on by Ulquiorra in ep.114 (@8min), when Zangetsu tries to take control of Ichigo. He states that the power of Ichigo's Bankai is "inconsequential", but when his Reiatsu spikes, it's stronger than Ulquiorra's own. This is also demonstrated in ep.269 (@17min), where Ulquiorra dominates Ichigo’s Bankai, but is forced to retreat when Ichigo dons his mask. It doesn't say how much stronger Visored get when they hollowfy, only that it's a significant increase, so let's assume it's half again that of their Bankai. This means that Ichigo, while hollowfied, potentially has ‘’’three times’’’ stronger Reiatsu, than the average captain’s Bankai.

Trying to compare power levels this way is difficult, since Ichigo isn’t as skilled as a captain, when it comes to using his massive Reiatsu. E.g. when Ulquiorra later released his Murciélago, he overpowered a Hollowfied Ichigo to the point where he was completely helpless to fight back. Toshiro, on the other hand, did quite well in his fight with Harribel, after she released her Tiburon in ep.274. In FKT, Aizen easily kept up with a hollowfied Ichigo, while only using his Shikai in ep.295 (@4min), and since the Bankai release generally boosts a Shinigami's power by five to ten times, and considering Ichigo’s immature Bankai, I’d say that Aizen was around five times stronger than Ichigo’s at the time, when comparing their basic strength. Remember that Aizen was a monster even before obtaining the Hogyoku, as Ichigo learned the hard way at Sokyoku Hill in ep.61 (@13min)

When Aizen later entered the “Crysalis stage” in ep.300 (@11min), he became unreadable to anyone save Ichigo, so one can only speculate how much stronger the Hogyoku made Aizen from then on. The only other reference point would be Ichigo, using Saigo no Getsuga Tensho, and since he destroyed mountains and shook off a full incantation Hado 90, as if swatting a fly in ep.308 (@20min), that’s really no help at all. That fight can’t be compared to anything else we’ve ever seen in the Bleach universe.