His highest power level was during the fight with Aizen, and he was the calmest he had ever been.
It was a suicide move, though. Does it really count as Ichigo being presented as powerful if it's with an ability that halfway through the fight nearly kills him and "permanently" takes his powers away?
EDIT: and even in that fight, Urahara's trap is the thing that actually beat Aizen. Ichigo was on the ground about to be stabbed right before it went off. Even in that form, he STILL didn't actually beat the person he was facing.
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"History Is Much Like An Endless Waltz. The Three Beats Of War, Peace And Revolution Continue On Forever." - Gundam Wing: Endless Waltz