I'm going to say Kobo and Sony eInk based readers. Once I'm reading, all of the eInk based devices are pretty much equivalent in my mind. Kobo and Sony have the lead over Kindle simply because they support ePub. I have never used a Nook so I cannot comment on it. I like reading on my Android tablet too, but sunlight readability and battery life are big considerations for me so they can never qualify as a favorite in my books.