My favorite books
As a kid, I'd say my favorite series was Magic Treehouse, solely because it gave me the feeling that I was ahead of other kids my age for being able to handle chapter books. The stories themselves were pretty straightforward, not too much overarching plot, and each story was pretty self-contained. Sure, at a certain point the stories moved away from where the series started and they were longer-winded self-contained adventures, but they were still well written for what they were.
When I was older, I read the third Harry Potter book at school, read the fourth, then the first two, and finally finished the series afterward. I was so obsessed with the series I made my parents find the only English language book store in Rome so we could get the last copy the day it came out... and picked up the last copy there. To this day I am a die hard Potter fan (Potterhead?), albeit excluding all the ret-conning that Rowling has been doing via Pottermore.
The Pendragon series was also a huge part of my childhood. It doesn't get the recognition and attention it deserves, but it was one of my favorites growing up because it felt so different to everything I had read or experienced up to that point. I tried to get my brother to get into it and he got about 4 pages in and stopped.
In terms of classic novels, though, Crime and Punishment takes the cake. Part of my respect for it stems from the fact that I was one of the only people that actually read it in my senior year English class in high school, and part stems from the talk about dreams and consciousness that the narrative grapples with. As a writer who likes to mess with the bizarre and its effect on reality at times, I had nothing but respect for Dostoyevksy's grasp of it.