Many under rated characters in the manga. Here's my list in order.

1. The Vizards( Seriously, ex-captains, which means they have Bankai and on top of that, with a hollow mask! Something like Ichigo, but must be much more crazy)

2. Kurosaki Isshin (I have a feeling that he might be an ex 'zero' squad member... Beat the "powered up version" of The Grand Fisher with one swing of unreleased Zampakuto not at full strength!!!)

3. Kisuke Urahara ( This ex-captain is speculated to have a very devastating Bankai! He once mentioned that his bankai is "not suited" for training... Hwta the hell is that supposed to mean?)

4. Shihouin Yoruichi (Remember, all captains, and ex-captains have a Zampakuto which can release bankai. *Zaraki Kenpachi is an exception to this rule. This means we haven't really seen her REAL strength besides her basic combat ability WITHOUT using any Zampakuto.)

5. Ishida Uryu's Dad (The rival/friend of Kurosaki Isshin should be a formidable person in every way! Sorry, for got the name... )

6. Yachiru Kusajishi (The pink haired wonder girl... I wonder how strong she really is... Remember, Ichigo commented on how fast she was while trying to follow in his bankai form...! Plus, we have never even seen any of her released forms)

7. Juushiro Ukitake (Just because he's sick, doesn't make him weak)

8. Shunsui Kyouraku (Lazy but strong guy)

9. Unohana Retsu (Never know what she might come up with)

10. Ichimaru Gin (Unpredictable, never seen his bankai, and possibly has a hollow mask...)

12. Kaname Tousen (He seems to be able to use a hollow mask like Ichigo and the Vizards. So, how strong can he really be?)

13. Sado Yasutora (Poor guy... Always meets overly strong opponents)

14. Inoue Orihime (With such a unique power, the possibilities are endless... If you think about it)

Id have to go with Isshin Kurosaki, Ichigos dad. I wonder how strong he really is and how important a role he will play later on in the story. There have been a few hints. Plus he is really funny!