For my favourite book, I've got a tough interplay between books I absolutely adored as a kid (but which are for children at the end of the day) and ones I've since read which stay with me more, though I can't say I enjoyed them as much as, say, Harry Potter.
The best books I read as a kid I would say are (in no particular order): Series of Unfortunate Events; The Tomorrow Series, His Dark Materials and, of course, Harry Potters. I also liked the Artemis Fowls, and, for whoever mentioned Redwall, I had the same infatuation until I realised so many of them are exactly the same story just with slightly different characters or settings.
More adult books I love are: The Count of Monte Cristo, a Tale of Two Cities and For Whom the Bell Tolls. I also really liked Metamorphosis and Nineteen Eight-Four (lucky I ended up studying it in Year 12).
I'm currently reading Lolita and the Great Gatsby, and had to look through the Prince by Machiaveli this week for Uni.