I chose MoonReader, but really I can't see a heckuva lot of difference between the 10 or so reader applications that I use. (MoonReader, Kindle, Readmill, Kobo, Google Play, Acrobat, Pocket, Firefox, FBReader, OfficeSuite).
What gripes me is that most of them want the books in a certain directory of their own, and it is not easy to organize books into categories. I really miss the file management capabilities of the desktop computer.
And they all have different interfaces and conventions - this is like the usability hell of personal computers decades ago.
On the plus side: Acrobat is excellent for PDF files. Kobo Reader has a nice system of Collections. OfficeSuite has the capability of jumping around the file system easily.