Was there any question? Gotta be Kindle. Can anything challenge Amazon’s Kindle app for playability, ease of use, and price? (free!) Whatever you download onto your Kindle is also available in your Kindle app on Windows, Mac, iPhone, android and/or all of the above. It’s genius.
I disagree, but I am ignorant of some of the new Kindle options. I prefer to use Calibre… it is free, and over 22 million people have used it. It just keeps getting better. See: http://calibre-ebook.com/ It works on my desktop, laptop, e-reader and smartphone.
It works great for the 1300+ books I carry around with me in electronic format. All of my books are free… so I stay away from devices that are hooked to sties like Amazon, Apple store etc. Many of my books I have scanned in myself (and cleared my bookshelves) so everything is now electronic. I have nearly 30,000 books on my desktop… all cataloged by Calibre without a hick-up, and all searchable so I can find things very quickly.