My favorite author is Terry Pratchett, but if I had to choose a favorite book, it'd probably be The Giver, by Lois Lowry. I know she's considered a children/young adult author, in that her language is pretty simple.. but the themes and the way they're expressed are heavy. It's the sequel/companion to Gathering Blue, which I've also read and is also very good.
It describes another world, one where everyone's lives are planned out for them in childhood, and there's no real choice or deviation. They can't even perceive color anymore, because color would give people more choice. "The Giver" is the one person in the village who has memories of another time, a time where there was love and color and choice, and it's a highly respected and important role in the community. Then a young boy is chosen to be the next Giver, and receives small snippets of memory in sessions with the Giver, but.. problems develop, and a dangerous plan is hatched. Excellent book! The kind that makes me really think every time I read it. Lowry's a really good author.
I also love The Chosen, The Tao of Pooh, The Woman in the Wall, aaaand yep just about anything by Pratchett, lol. I think I've read The Hogfather about five times.