Feats shown Juha is the only one that is able to defeat him... He cut down Bankai Ichigo with little to no effort... Also Gremmy Thoumeaux his powers are in his Imagination he's pretty op as well...From what we know only Shinigamis battles are battles of Reiatsu, Hyogaku Aizen is no longer Shinigami and Hashwalth is Quincy, so power of reiatsu don't apply here.
Schrift Hashwalth by hype should overpowered Shinigami Aizen and with Vollstandig he should be able to keep up with two or three Hyogaku Aizen's form, but for know it's his limit, of course until we get more of him. Balance power have some limit, otherwise Haschwalth would be unbeatable.
Actually no, Yamamoto was able to found "weakest" and counter KS.he maybe able to defeat aizen,if he doesn't take a look at kyouka suigetsu...remember once seen you fall under complete hypnosis....with no way out.
Aizen didn't beat Yamamoto's bankai, Aizen didn't even beat Yamamoto's shikai, Margela was the one who stopped Yamamoto.Didnt Yamamoto get solo'd by Aizen then Ichigo had to beat him? Not to mention but I dont think juha bach can use yamamoto's bankai better then him so I think Azien has a shot to beat him but we all know who is going to beat him ICHIGO!!!!! YAY!!!! or Isshin or Kenpachi
Aizen is no longer transcendent, behold:We are forgetting the crucial problem.Aizen is a transcendent being(shinigami,zanpakuto and hollow),while Yhwach is a quincy.He is at a higher dimension of power than everyone except Ichigo.As for Yamamoto's bankai , i think it's not such a special thing.Aizen's fragor created a crater kilometres in diameter.
Yhwach is the most powerful quincy but his greatest weakness is his complex soul power.
I think that's just the new form he took after Ichigo cut him with Mugetsu.Aizen is no longer transcendent, behold:
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his right eye is now normal and his left eye is purple now:
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His left eye is normal too, so his back to base mode but just with immortality