Shunsui and Unohana are the two strongest. Closely followed by Ukitake. Ukitake has a weak body, so as well as randomly having moments of illness, he may be more susceptible to damage...
I feel the moment Shunsui beat Starrk, and seeing Starkk take out two captain class Vizards, that when he was in the mood for it, Shunsui was a different level, and after he lectured them on a duty as a captain, to ignore style, you knew he was a senior captain.
We've never seen Unohana fighting at her best either, so maybe when the time comes, we'll see what they're made of.

Kenpachi, Unohana, Shunsui, Isshin and Urahara are the 5 characters I want to see fighting at full power. I imagine they'll be nothing short of awesome :D