Oh man...do I have to give just one ;)
My favorite book is Treasure Island by Robert Louis Stevenson. But that's for sentimental reasons, really. It's the first book that really got me into reading as a kid.
The Road by Cormac McCarthy is a book I always recommend to people, and I've never gotten a negative review from those who have read it. It's a great story set in a post-apocalyptic world and focuses on the relationship between a father and son as they do everything they can to survive. It's heartbreaking and brilliant.
I could go on and on but you only asked for one :)