Board Thread:Questions and Answers/@comment-25784442-20141130034710/@comment-5398287-20141130173502

Well, there's always Yama's oft-referenced "strongest Shinigami in a thousand years" boast. If Ichibei was born before the Gotei 13 was founded, which is very likely given his naming ability, then there's a good chance he's stronger Reiryoku/Reiatsu-wise than Yamamoto was. In terms of raw power and abilities, he's definitely up there, but so far we haven't seen anything akin to Yamamoto's Shikai being used, his Hakuda during his fight against Wonderweiss, his ability and skill to take on the two strongest captains at once and come out unscathed, etc., so it's hard to base anything on that.