Yamamoto, the captain commander of the gotei 13. He said that there hasn't been a shinigami stronger than him born in the past thousand years. Aizen may be stronger than Yamamoto, but the only reason Aizen defeated the captain-commander is because yamamoto absorbed his own zanpakuto to save Karakura town, so yamamoto is probable stronger. It could also be Ichigo because Unohana noted that even with only a sleeve of his Bankai Shihaksho he was at captain level reiatsu. And Ichigo said that his Shihaksho is tied to his reiatsu, so the more in tacked it is means the more reiatsu he has.