It was very tough for me to choose. I chose A Game of Thrones, mostly because there was so much action, and Martin set up the characters marvelously. He made me get attached to so many characters, then took them away. I remember being pissed while reading it, but it really set the tone of the entire series as well as any first book in any series I've read. Storm of Swords is right there though, what an amazing book.