A very difficult question as I have read and enjoyed so many books over the years... but if I had to choose just one book it would be "The Time Traveler's Wife" by Audrey Niffenegger.
Forget the film - although they made a brave attempt - it was never going to come close to the book! I would heartily recommend it to anyone... Except maybe an aging aunt or grandmother. There are a couple of scenes I can think of (that were not in the film) that would shock some people!
I also loved Weaveworld by Clive Barker, The Mists of Avalon (and the rest of the series) by Marion Zimmer Bradley, Dagger Spell (and the rest of that series) by Katherine Kerr and Clan of the Cave Bear (ditto) by Jean M Auel... and, er... I'd better stop now.
Whatever you do, please don't ask us to name our favourite film!