I don't really have a single favourite book, but in the event of a fire at home, the first book I'd save is my old Collins edition of Three Men in a Boat - bought with Christmas money after I'd seen the film on TV, and it's been with me wherever I go ever since (and yes it still makes me laugh 50 years on).
After that - an illustrated book of poetry which was a wedding present from my wife, then my battered paperback copies of The Annotated Alice and The Annotated Snark. Then one of my illustrated copies of the Rubaiyat of Omar Khayyam - the one published the same year my father and father-in-law were born.
After those ... well it could be a long list ! And while those choices may seem to be all sentimental, I do read them all from time to time and enjoy them as much today as the day I originally bought them.