What gets me is the whole thing about Aizen saying Yama is stronger. I think people are taking it the wrong way, I think that for one Aizen saying Yamaji had the strongest zan, is speaking of his destructive power which was made clear from the start of the series, no shock there. As far as him saying Yamaji would probably win in a fair fight, I take that to mean that using just shinigami abilities & zans, Yama would probably win. Don't get me wrong, before chapter 391, I would have believed that Yama was stronger than Aizen, just that Aizen was the 2nd strongest & had so much hax. Now though, after Gins words (which to me is Kubo speaking directly to since Gin didn't say this to anyone else to scare them, he was talking to himself, so why lie), I believe Aizens full powers after whatever he did to himself, puts him on a whole different level. People ask why he'd create WW for Ryujin Jakka, well I'd say because though he is so powerful, Yama's hell fire could still him. The same way if you have a knife & I have a gun, you can still hurt me if I slip. Even more I'd say Aizen is saving his trump card for the Soul King & the RG. I believe Yama is the 2nd strongest up to this point tho (who knows about Urahara & Isshin), & I think we'll see his skill in the 4 arts beat WW before he dies.