Very difficult question. Ihave started reading "the Idiot" ande other classics just to realize I didn't like them at all.
I think if I had to pick one book that has really affected me it would have to be 1984. The last sentence in that book is so well written and has such a sad message that really moved me..
Other favorites are "twenty thousand leagues under the sea" simply because it's a fascinating book and very interestng to read and realize how close many of his views upon the future actually were to what really happened. I'm also a big fan of Douglas Adams and the "hitch-hiker"-series in particular.