Really, really hard, but if I were to choose an absolute number 1 I think I'd opt for Star of the Sea by Joseph O'Conner but all of these would be hot on the heels of that for me and in no particular order, Middlesex - Jeffrey Eugenides, The Blind Assassin and Alias Grace Margaret Atwood, The Poisonwood Bible Barbara Kingsolver, The Crimson Petal and The White - Michel Faber, Life after Life - Kate Atkinson, The Quincunx - Charles Palliser, Angela's Ashes - Frank McCourt, The Goldfinch - Donna Tart, The Heart's Invisible Furies - John Boyne, Shadow of the Wind Carlos Ruiz Zafon. Asta's Book, Barbara Vine, The American Boy Andrew Taylor. I'probably read these in the past 20 years or so but to go back further, I can remember my mother passing me a tome of hers when I was aged around 15 moaning about being bored, that tome was Gone with the Wind. I remember feeling pretty bereft when I finished it. In my 20s, maybe when I wasn't reading quite as much, on holiday I read The Thorn Birds and that also stayed with me long after I'd finished it.
So to answer the question of "why" it's that, books that stay with you long after finishing , feeling so immersed and lost in them that you never want the book to end and then feeling bereft when inevitably it does and knowing possibly it will be a long time before finding anything as good again. I always have a book on the go some will be great, many forgettable but that extra special quality only comes along infrequently there's a kind of deep satisfaction when it hits you at some stage into it knowing this is going to be a special one and afterwards you want to tell others about it.
Favourite childhood book The Water Babies Charles Kingsley.
I'll kick myself if I've forgotten anything.