I like Moon+ Reader.
http://www.moondownload.com/
Firstly, because it handles: txt, html, epub, pdf, mobi, umd, fb2, chm, cbr, cbz, rar, zip or OPDS - which means I don't have to convert books after I strip the DRM.
Secondly, because it allows me a great degree of fine control over how the book appears - from fonts to font size to background to etc.
I never use the native Nook, Kindle or Kobo readers simply because they only have a limited number of font sizes, and the larger increments after the mid-size font are just too drastic. I need slightly larger than the mid-size font, not top-line eye-chart large. They also usually just give you a choice of white, sepia or black backgrounds. I prefer a faint green color, it's easier on my eyes.
Forgot to add, it also lets me access ebooks I've got stored on my NOOK HD's external SD card.