Anathem by Neil Stephenson
Anathem is the smartest book I have ever read. That isn't to say it's the best book I've ever read, but every page drips with philosophy and science. What makes Anathem incredible, though, is the skill with which Neal Stephenson incorporates the ideas into the flow of the story.
The book is about an order of monks dedicated to science rather than God, and cloistered by the world for fear of what they might accomplish were they unrestricted in their pursuit of knowledge. The book is full of platonic ideas, quantum mechanics and discussions of consciousness. There are times when it can seem like Stephenson is just having his characters talk about an idea simply for the heck of it, but in the end almost every philosophical dialogue has an important impact on the story. Stephenson is smart, and he assumes the reader will be able to keep up with him, which probably means it's not for everyone.
Really, Anathem could serve as an introduction to philosophy and basic particle physics were it not for the fact that Stephenson changes the names of everything (for example, Occam's Razor is Gardan's Steelyard). The density of philosophy in this book made it highly pleasurable for me as a philosophy major. The only issue I have with the philosophy of the book is that it's rather anti-religion, but that's just personal.
In the end, however, the point of a novel is to tell a story, and in this Anathem also succeeds. It's not the greatest story ever told, but it's compelling and even mostly believable.
Stephenson's writing style is a bit frustrating at times, his descriptions of things can be hard to follow and the characters memories of things sometimes seem rather distinct for a first person account.
In the end, I recommend this book to anyone who loves philosophy, ideas and science fiction and if I don't miss my guess this one will be winning the hugo and nebula awards this year.