Well, I can't get Nook as I'm in Scotland.
Shubook I've played around a bit with the free version but don't really like.
Bluefire is good but I miss having a dictionary.
iBooks is the best but I like having more customisation options...I don't think there is an app that's as good as Aldiko or Cool Reader on Android. Stanza was, and was working fine on my 4th Gen. but not the new Touch.