The obvious one would be 1984. I don't think anybody who reads this can really help but see the world differently.
In the Skin of a Lion by Michael Ondaatje was really a big one for me. It also turned me on to The English Patient, also by Ondaatje, which remains one of my favourites to this day. Really everything by Ondaatje captures the essence of the human condition, and translates it in a way that is poetic beyond belief.
In terms of non-fiction, A People's History of the United States was THE definitive book which really interested me in both history and politics and bridged the gap between ignorant youth and conscientious young adult-hood.