[DarthMischa]

While Shunsui is my favorite, I'm going to say Unohana, not because of anything we've yet seen, but what we've not seen. This based purely on a story telling point of view. Anime like this progress fairly predictably: the sooner someone's powers are revealed, the weaker they are in comparison to those who remain mysterious. We've seen something of what Kyouraku can do, and even less of Ukitake. Unohana, on the other hand, we've only seen as a healer. The fights all progress up, like with the Espada. I think that the formula will reveal that she's going to be one heck of a scary opponent.