The book that most exceeded my expectations: The Emperor’s New Mind, by Roger Penrose.
Even though its conclusions are almost certainly dead wrong, it’s interesting and mind-expanding throughout. This book caused more deep thinking in me than any other.
The most disappointing book: Gödel, Escher, Bach, by Douglas Hofstadter.
An overlong, boring hodgepodge of ideas that I either learned and/or occurred to me independently by the time I was twelve. My reaction on finishing this book amounted to, “That was all? That was fucking all? 700 pages that seemed like 7,000 for that?”