What is your favourite novel, and why?

I was asked this question yesterday (at a literary event), and my mind just went blank. I grasped at straws, and said Great Expectations, which is a very good book, but probably not my favourite of all time. Coming home on the bus, I started to think about what I would say if someone asked me again, but I'm not much further forward really.

How would you answer that question? Do you have a favourite novel, and do you know why you love it? If you can't make up your mind, what are your top three (or four or five, if that's easier)? You can change your mind tomorrow, so don't let the question faze you like it did me .

My list would probably include:
Maus by Art Speigleman, although maybe that shouldn't count, as it is a graphic novel
The Women's Room by Marilyn French, although it is probably terribly dated.
The Woman Who Walked Into Doors, or pretty much anything by Roddy Doyle, who is the only male author I know who can write convincingly from the point of view of a woman, but I've changed my mind already writing that (other contenders are The Autobiography of Henry VIII by Margaret George, Life of Pi by Yann Martel and The Kite Runner by Khaled Hosseini) and as soon as I see other people's choices I will change it again.

what are yours?