The Da Vinci Code keeps a frenetic pace that makes "quality" secondary, and quite difficult to achieve. (I would submit that The Bourne Supremacy is superior in both pacing and quality, however).
Harry Potter is insanely accessible and easy to read, and has a very surprising amount of internal consistency, even over 7 books (which is in stark contrast to most mainstream sci-fi/fantasy series).
As for Twilight, I don't know what makes a book appeal to that demographic, but it seems pretty different than any area I've dabbled in as a reader.
(I worry that giving opinions like these opens me up for some interesting flaming, but whatever.
