This is a hard one. So many books to choose from and which one to commit as best one ever, ever. Favorite author is hard enough, but best book is just very very hard. I loved Tralarna, Kulla-Gullla, Pollyanna as a child, the Hobbit (and all the followers) in my teens, The World According to Garp followed and then The Master & Margarita. Cat's Eye and Anna, Hanna & Johanna I cherished reading. Finding new authors (new to me, that is) and reading all their books led me to Charlaine Harris - I do love Dead Until Dark, and Ali Brookmyre. Getting recommendations from friends helped me find the excellent Lamb: The Gospel According to Biff and World War Z. The bookclub led me to Shantaram and a lot of other books that didnt quite make this list.
But if I have to, and it seems I do, I will have to say that my favorite book of all time is Birdsong by Sebastian Faulks.