I guess when I read for enlightenment and knowledge, I absorb in a way that I rarely think about the book itself... and they all commingle and combine to create my own personal perspective and awareness of the world around me... the universe I live it.

When I read for entertainment, with rare exceptions, once the book is done, it's done. Even those I poured through in short order... barely more than an entertaining movie.

So my 'favorite' has to be those stories that bring me back to reread over and over again.

The Lensman Series - Seven books by E.E. Doc Smith. Space opera from the 1930's.

and

Ender's Game and Ender's Shadow by Orson Scott Card.

I enjoy them afresh, every time I crack them open.