Board Thread:Questions and Answers/@comment-10952853-20130617015352/@comment-4800978-20130617022134

Canonically the strongest was Yamamoto, who Aizen himself admitted was on an entirely different level to himself. However Yhwach could contest that claim, if it wasn't for their 'battle' being manipulated beyond a straight 1 v. 1 match-up. We don't even know what the Soul King is capable of, only that he is considered vital to the Soul Society's continuation.