This is a really difficult question, but I'll say The Catcher in the Rye, if only for sentimental value. This is a book that I read first when I was 19, after having hearing some people saying how good it was, how the book changed their lives, etc. I think the book also changed my life, and at the moment of reading it I considered it the best I'd ever read. But hence I've read a lot of other books that arguably I could consider better, maybe some by Stephen King, others by Michael Crichton, Dean Koontz or Brian Keene, even the Harry Potter series. But in this case the heart wins over the brain and I give my kudos to Sailinger.