The title says all, what is your absolutely favorite book? Lets keep it simple; it can be anything in circulation (i.e. printed or electronically published.)
Mine would have to definitely be Hitchhiker's Guide To The Galaxy series by Douglas Adams. What can I say, it is a classic. The book itself is very humorous and satirical, with parodies of everything from bureaucrats to religions. The book has everything from maniacally depressed robots to a species of fish that disproves the existence of God. If you haven't read the series yet, I recommend you definitely pick it up.