Mantano! I've been using it since the beginning. It does everything a reader app should. I do subscribe to their Cloud service which is awesome!
I've tried Moon+, Cool Reader, Aldiko, FBReader and a few others and Mantano is the one one that reads external style sheets correctly and still allows you to customize your reading view.
It has collections and you can sort by tags, series, author, title....etc It's well worth the price to buy the full Premium version.
Mantano is the only reader app that has crop for PDFs which is great when you are trying to read one.
I use Mantano with my Dropbox app. I export the books to my epub folder on my device from Dropbox then import them through the app and sort into collections.
It's more than a ereader app it's like having a whole shiny new eReader! I use mine on my new Kindle Fire HDX which equals bliss!
It also supports custom fonts just not custom bold or italic text that shows in the default font still. It is the only issue I have with it. I think Cool Reader is the only one that does allow for italic and bold custom fonts.