Nineteen Eighty-Four and The Curious Incident of the Dog in the Night-Time are both
great books. I can't choose which one I'd prefer, so I'm not going to vote.
If you want to read a classic-ish kind of text I'd pick 1984. 1984 is probably one of those books that will blow your mind and make you think about things a bit differently. There is a lot of depth to this book too, so heaps and heaps of material to analyse.
If you want to read something a bit newer pick Curious Incident. It'd rank up there with some of my favourite books. It's about this autistic boy Chris basically trying to deal with his family problems. That probably doesn't sound that fascinating but the main character is very interesting and the book is packed with a good sense of humour.
When do you have to pick this by? If it's before the start of next year, honestly I'd just read both and take your pick. Otherwise, just take a look at the excerpts:
http://books.google.com.au/books?id=mZbVwG8f1ucC&printsec=frontcoverhttp://books.google.com.au/books?id=w-rb62wiFAwC&printsec=frontcoveredit: sorry gave the wrong link, i accidentally linked the play version of Curious Incident