Board Thread:Questions and Answers/@comment-10952853-20130617015352/@comment-4800978-20130617035458

You can't say that Yhwach is the strongest as Yamamoto used a large amount of his power fighting Royd, only for Yhwach to then turn up and steal Yama's Bankai. It wasn't a fair fight, and as such shouldn't be judged as such. We only know Yhwach is strong enough to defeat Yamamoto without the latter using his Zanpakuto.