Not sure if these two have been mentioned but meh, I'll mention them.
First, my favourite book, is Ulysses by James Joyce. This is not your "ordinary" fiction so if you are just a casual reader or w/e this might not be the book for you. However if you enjoy reading, and like a tricky book that is hard to grasp, this is the book for you. There are so many things to discover while reading this, that even if you read it 100 times you wouldn't get them all.
Secondly, I would like to recommend The Trial by Franz Kafka. Again, this might not be your ordinary kind of book, so before you read it make sure you do your homework and get a little information about it first; it's a tough book to read, but a very rewarding experience.