I never really like this question, because I never have a real answer to it and I don't like to say: "there so many great books, so I can't choose". So my answer changes almost every day. But there are some books/series which I think are spectacular:
His Dark Materials by Philip Pullman
There are some books everybody should've read and this is one of them. I haven't met a person yet who could honestly say about this book that they didn't like it. And for me it's impossible to imagine. You have to like this book. First of all, because it can be read by everyone from every age. The story has so many layers, so not everyone will read the same story. And that's what I love most about this book.
The Time Traveler's Wife by Audrey Niffenegger
This is the book I'm still most impressed by. I can't say exactly why, but it just amazes me. Audrey Niffenegger made a complex work about time traveling. For me, it has all the facets I could want in a book: it makes me cry and laugh. It has romance and suspense. It's my personal favorite, but I don't think everyone will like it.
Of all the series I'm currently reading this is the best. This is already a beautiful year, because the next book will be published, City of Swords. And I can't wait. Every two years there's a new book and every time the same things happens: I run (almost literary) to the bookstore, buy it, read it, love it and then I have to wait another two years for the next book to come out. The books can be read separately, because in every book there is another principal character. In the Netherlands it's a series that isn't really popular, but it should be!
So, what's your favorite?