I tried a bunch of eReaders on a recent Android test device and I found that I actually like the Kindle reader best for MOBI/PRC format books (Kindle device format). The trick is how to load books into it that you didn’t buy from Amazon.

• So once you open Kindle App on the device and it syncs downloading a book or two (at least the user guide and dictionary), reboot the device.
• Now attach it to your computer and find the Kindle folder (usually on SDCard or “Internal Storage” depending on your device). If you don’t see a Kindle folder, create it (with capital K). 
• Now add your mobi or prc files into that folder.
• Unplug the device from your computer and reboot it again. The books should show up now and look fantastic. I recommend the sepia option for background color.

There aren’t a lot of ereader options for MOBI format so this worked great for me. If you use ePub though, you have lots of good options.