What can I say, I had a bit of spare time on my hands so I thought I would attempt it lol. You can say which ones you prefer if you wish, I don't really mind.
I basically said my list, or at least a short list of the top characters. Some people haven't shown all of their abilities, but the top people seem pretty obvious: Aizen, Yamamoto, Ichigo, Isshin, etc. Others like Gin, Urahara, are pretty near the top too. Some of the other top captains are strong but don't seem to be a match for someone like Aizen.
At the point I'm at in the manga, the number of important characters seems to have dropped significantly.
Cool, I assumed Gin and Tousen are probably actually stronger than what they seem to be at the moment. It will be good to see the stronger people fight. By the way have you seen all the captain's bankai where you are?
Not all of the bankai have been revealed.
Aizen hasn't used any sort of bankai yet, but has shown something much greater.
Yamamoto apparently hasn't used his bankai yet even though at one point he does fight all-out. Same with Urahara, he fights all-out but doesn't use bankai.
Shinji reveals a pretty good shikai but not a bankai, again, even though he fights with everything he's got.
Unohana hasn't revealed her bankai, and neither has Ukitake. Kenpachi doesn't have a bankai.
Probably the only bankai I know about that you don't know is Gin's. He shows a pretty awesome bankai. But it's still hard to tell how strong Gin really is. He was a genius in shinigami school where he finished in one year and his bankai is pretty epic.
-Lyn