Mine's a tough call; it's either "The Brothers Karamazov" by Dostoyevsky, or "Watership Down" by Richard Adams. The latter has been my sentimental favorite since I was about 12 (I've re-read it about six or seven times now and still love it just as much), but man, Dostoyevsky is just sheer genius, I don't even know.
Somewhere behind those two is "The Golden Compass" (or "Northern Lights" as it was originally known) by Philip Pullman. I didn't like the other two books in the trilogy as much but man the first was great.