I find Alreader the best as it can open a wide range of files (epub/mobi/azw/fb2/html/doc etc) I downloaded it for free in the G store; no ads either. I think it cant open pdf tho. Something you might want to try
My experience on FBReader is superb. You can search for it at Play store. But I would suggest you to find another app for PDF, though FBReader do support PDF through a plugin, it is quite horrible to read PDF in an ebook reader.
I like Moon Reader +. Only issue to me is it does not display children's book with a lot or large pictures on a page. Other than that, i have used it for many years. Thanks Mobilism!