I hate having to narrow myself down to one book! I suppose though, like SidKemp, I would pick Tolkien's Lord of the Rings trilogy.
I love this story, because it deals with all the major questions that concern humans, but set them in the context of fantastic adventures. Apart of issues of love and hate, good and evil, the problem of war, Tolkien was very much ahead of his time in treating questions of the environment and ecology, making pollution and genetic engineering part of the evil that emerged from Mordor.
Despite using a fantasy/fairy tale setting, Tolkien's characters are complex individuals with many internal conflicts. As a result, it is easy to identify with them and feel for them.
Tolkien created a complete world with its own languages and legends, which can be further explored in his other books, essays and notes. This gives another feature that I love about his writing. When I read him, I enter his world fully. This element of escape is very attractive to me.
I first read the Lord of the Rings when I was 11. For the next ten years or so, I re-read it at least once a year. I read it out loud over a number of months as a bedtime story to my children. I hope to find time to read it again soon.