I don't even have a frame of reference for tackling this one. The concept of "favorite" book just bounces right off my brain. There are just way too many to choose from. So here's a few:
The Phantom Tollbooth - I adore this book. Every last word of it makes me smile
The Two Princesses of Bamarre - Makes me cry every.single.time
Dragon Singer - My first introduction to Pern, so it has sentimental value
The Poisonwood Bible - Not without its flaws, but the only book I've read that pulled off the "more than one narrator" trick with any believability
Black Thorn, White Rose - Like Dragon Singer, this was my first introduction to a series. It was also the first book I ever read that I didn't put down without immediately thinking, "I could have written that"