So many people in here are making it sound like the only books they've ever read are the ones they were forced to in high school...
I mainly like Sci-Fi, I'll second Japan77 and pick The Foundation Trilogy. Revelation Space also ranks pretty high on my list.
I also like fantasy but I haven't read enough to really pick a favorite. I know a few people here are fans of a series called the Swornbright Archons or something like that. I think the first book is called The Wave Kings, it might be worth checking out.