I know you only asked for one, but...
By a long margin, my favorite book is Running with Scissors by Augusten Burroughs. I also loved Dry and Sellevision, also by Burroughs.
Right behind those is Cormac McCarthy's The Road and Kingsolver's Poisonwood Bible.
Yep, I'm a bestseller junkie, but I don't mind. They're bestsellers for a reason.
Other contemporary favorites include: The Lovely Bones, The Time Traveler's Wife, The Memory Keeper's Daughter.
Growing up, my favorites were: The Outsiders, Lord of the Flies, The Chocolate War, My Side of the Mountain, Where the Red Fern Grows, Gone with the Wind, The Hounds of the Morrigan, and Little Women, among hundreds of others.
And while I read the whole thing, I had a hard time getting through The Historian. It dragged and dragged. It's one of the only books I've read that I wouldn't consider "good" that I have actually completed.