Not to be a complete Potterhead, but I'll have to go with Harry Potter again. There are plenty of other series that I love (including His Dark Materials, The Hunger Games, etc.), but Harry Potter was just such a big part of my childhood (so cliche, right?) that I have to pick it. There is no other series that I read for nine years of my life, and there is no other series in which I believe the characters and fictional world they live in are so well developed, which allowed me to identify with the books more.