Kindle has a good app. Google Books is ok but you can't do any useful organizing. Kindle will need an Amazon account. The Kindle hardware reader is easier on the eyes too if you want both a tablet and a dedicated reader. You can go back and forth without losing your place.
Depends on what format and where you are getting your books. I like Moon+. It handles all formats including PDF (Pro Version required for PDF). It doesn't handle DRM books. There is a learning curve but several very good internet forums with help from users. If you are buying from Amazon use Kindle app. Then you don't have to worry about DRM on their books. Barnes and Noble's Nook app is handles their books and other epub and PDF books. Personally I don't care for Google Books for several reasons not the least of which is they want to store every book in their Cloud.