My absolute favourite book of all time is Watership Down, but I recognise that isn't for everyone. I'm not sure if you're looking for fantasy in particular, or are open to other genres so I'm just going to list some books I enjoy for one reason or another:
Phantom of the Opera: Just a wonderful piece of trashy Gothic horror/romance written by a madman.
Woman in White: Or pretty much anything by Wilkie Collins (the inventor of the modern detective novel).
Fahrenheit 451 has been mentioned, but I'll add The Martian Chronicles to that.
The Raven Saga by Giles Kristian is good. Vikings, sex, gore and silver.
Anything by Terry Pratchett, take your pick. Even the (terrifying) children's books are good.
It's a cliché, but I really do like 1984.
CM Priest's Clockwork Century books are excellent, Steampunk horror-y goodness.
And for luck, a book I like by an author I hate: American Gods. However, I strongly recommend reading Douglas Adams' The Long Dark Teatime of the Soul straight after.
If I think of more, I'll come back.