This is a hard one since I’ve read so many and yet there are still many more I haven’t read. For now, it’s Good Omens by Neil Gaiman and Terry Pratchett. Why? It’s the book that shows that the end of the world can be funny. I actually believe it’s a YA novel, but you’re welcome to dispute me on that.
To be fair, I believe that if a novel elicits emotions, it’s automatically a favorite for me. If I cry, if I swoon, if I laugh, if I argue with its characters, it’s a favorite book!