I really like Play Books, just for their ability to upload books and have them sync to any device, even read them on the web without the app itself.
Another app I liked was Kobo, especially the widgets and all the info it provided on reading stats.
Many others will, no doubt, recommend Moon Reader, although I never used it because it couldn't render a custom-made ePub I had (it had color formatting on the text, and it just displayed black).