Board Thread:Questions and Answers/@comment-10952853-20130617015352/@comment-5398287-20150421184457

Most technically, Ichibei looks to be the strongest of them all. He's the leader of the Royal Guard, who are effectively the elite captains among all captains, and given how he effortlessly trounced Yhwach again and again during their battle even prior to revealing his utterly insane power to remove, change, and assign names, he's certainly proven his power. It's logical to assume that he stands outside Yamamoto's "no one stronger in the last 1000 years" claim because he's definitely been around since before the Gotei 13 even started (having had to name things and all), so I'd say he am the strongest.