Can't decide! I'm not counting iBooks, although technically it is a download. But it feels to me more like part of the basic functionality of the device.
Netflix (assuming it's not a really mean April Fool's joke) and Pages would definitely be candidates. Also, there look to be some cool music-related apps that will be available, such as a virtual keyboard based instrument that looks nice. Will the Kindle app be available by the time I get mine (late April)? If so, that would give me immediate access to my Kindle books, so that might be the most practical choice. Hmmm. I can download more than one, right?