Platform: Microsoft
Every phone or tablet should have an eBook reader. While there are plenty of options on Android and iOS, Microsoft has been a little late to the party. Tucan Reader is no Aldiko or Moon+ Reader (arguably one of the best eBook readers on Android), but it's pretty good in its own right. Boasting a decent UI and a some rich features, Tucan Reader is one of the best eBook reader apps on Windows and 
it's free.

Tucan Reader can import books to the library from phone, SD card, OneDrive, Dropbox, Google Drive, BOX etc.  The supported formats include HTML, FB2.zip, PDF, MOBI (DRM-free), ePub (DRM-free) and TXT. It also has the ability to open ZIP and RAR files which is pretty neat. It has most of the features you would expect from a decent eBook reader such as brightness control, day and night modes, font and paragraph settings, search string in book, etc. 

The app works on both Windows Phone 8 and 8.1. The file size is a bit on the heavy side with 20 MB.
Pros: Plenty of options and features, decent UI, can read most popular eBook formats
Cons: No separate library, too many features make the interface slightly distracting.
Verdict: I think the eBook readers on Android and iOS are still better but on Windows, this is as good as it gets. Reading with this app has been just as enjoyable as with any other app on any OS for me.
You may also try Bookvisor, Kindle, Legimi.