I've read ~25 books from iBooks, Kindle, and Nook and I prefer the look and feel of the iBooks. However, the Kindle app has a better selection and seems to sync between my Touch and iPad more reliably. I also find them somewhat cheaper.
To be honest, I've only used Nook a couple times, but it looks fine, too.
To be honest, I've only used Nook a couple times, but it looks fine, too.