Board Thread:Questions and Answers/@comment-5775318-20130223175459/@comment-11051098-20140307210132

Yamamoto was the strongest Shinigami, but you have to take his Zanpautô into consideration.

He was skilled, if not a master, in everything a Shinigami can learn, he could probably manage to defeat every single captain bare-handed. But Zanka no Tachi is the factor that really made him - literally - invincible. Even Yhwach probably had to avoid Zanka no Tachi by stealing it, to be able to kill him.