Which science fiction book you would give to a first-time SF reader?

It only takes one great science fiction or fantasy book to get someone hooked. If it's the right book, that is. When you're faced with someone who likes to read but hasn't delved much into science fiction or fantasy, what the "gateway drug" book you offer him or her? What's the best book for a newbie SF reader?

Please include a book cover or other image so we can admire your choice!

Top image: Book cover art by Chris Moore.