My favorite is Power Support and it's the easiest to install IMHO. It's also a brand that is carried in the Apple stores, so they basically endorse the product . Unfortunately although there is a new screen coming out for the iPad2, it's not out yet.
Zagg is the last brand I would use. There are many reports of the Zagg protectors not fitting properly in their rush to get it to market. There are several threads on this already