Hmm, Most disturbing read must have been 'The Collector' By John Fowles. I remember feeling dirty by the end of the book, and the feeling of it wouldn't leave me for some days.
Another one that really stuck me was 'Disgrace' By J.M. Coatzee, both of these books left a cloud over my head. Still though, both of them are some of my favorites, as well as 1984.
A friend of mine just lent me The Collected Works of Marquis de Sade; not by request mind you. But i haven't had the nerve to begin reading any of it. I'm not sure how much moral depravity i want to submit myself to.