In my opinion, the Amazon Kindle is the best e-reader because of its two month battery life, the incredible e-ink screen, access to roughly one million books available for purchase, 3g/wifi, and much more.
I use my Tablet PC. I can purchase books from any of the e-Book providers plus I can download any physical book I buy at a bookstore in the form of PDF. The battery life is not bad either it usually takes around three days of reading before I have to charge.