I can’t name a favorite book, I love too many.
The Moon is a Harsh Mistress, by Robert A Heinlein was a great book.
Stranger in a Strange Land, same author, is one book I would identify as a favorite. In fact I’m going to go start reading it again even though I just finished it recently. Highly highly recommend it.
Good Omens by Neil Gaiman and Terry Pratchett is one of the funniest books I’ve ever written and I can only hope for more collaboration of the two in the future.