In terms of raw unadulterated power, I think Kenpachi might actually be the very top of the list in Bleach's powar levulz so far. The problem is he had little or no means of directing, focusing, or channeling that power, so the vast majority of it ends up leaking out into nowhere in particular. He has no formal technique, no grounded understanding of his own soul, and certainly no affinity for synchronization. He does everything by experience and instinct, which can be rather effective (especially in real life) but has its limits. He's about as efficient as a combustion engine if you ask me. I'd say far less than 10% of his power is able to be used in any functional way.
Ichigo beat him. Despite having an extremely high amount of raw power himself, Ichigo's doesn't really compare to Kenpachi's, at least it didn't during the Soul Society arc. But Ichigo had the benefit of being synchronized with his soul (Zangetsu), and possibly a little bit of that formal training from Urohara. Plus, Ken didn't give it his all because of that cliche reason of wanting to foster the development of somebody with the potential to offer him a very enjoyable battle.