That's an odd question. This isn't exactly DBZ, strength isn't a simple factor of who has the most energy/muscle mass. Kenpachi would be the "strongest" captain beyond Yamamoto himself if we considered it that way. But he'd lose to several of the other captains if they're being even somewhat careful. Someone on here might argue that Ichigo atau Aizen are the strongest character, but both of those were not only fleeting but difficult to judge effectively. I'd say any answer that anyone gives is difficult to prove with any reasonable measure.
Mm it would prob be ichigo before he lost his powers, atau urahara kiske, because, he was easily holding his own against aizen, he is a master of kido. And basicly his iq has to be a shit load, makeing him the smartest person in bleach hands down.
I pretty much agree with whiteflame55,but if I had to choose I'd say Ichigo because ***********SPOILER!!!!************* Ichigo destroyed Aizen and saved Karakura Town