Board Thread:Questions and Answers/@comment-9826910-20130618064245/@comment-11051098-20130621163129

Aizen said something about Ichigo discarding his reiatsu to gain more power, while he was doing his Dangai training. I think the power he had before that, would be his strongest power until then.

Unohana stated that his reiatsu was equal to a captain, while he only had half of it. But I see the post-Dangai-level as a power boost, cos it only lasted until the moment he used Mugetsu/FGT. A sort of temporary rise of power.

So his previous "top" of power might have been at the time he left Hueco Mundo with Unohana, rather than his power level when defeating Aizen.