Un) The Wizard of Oz - This is my favorite book of all time. In fact, I probably read through the original about once a year. It spawned 12 sequels written by L. Frank Baum, as well as countless Oz stories written by other authors. And how many times has it been made into movies? Plays? TV shows? Yeah... it's that good.
Deux) Harry Potter - Don't make me pick just one. The entire series is phenomenal from start to finish. And it's even more fun to read the second time because there are so many little things that you can pick up on that you may not have gotten the first time around.
Trois) The Stand - I may have had some issues with the way things ended in this book, but it can't be denied that the scope of the story is epic.
Quatre) The Hunger Games - The entire trilogy is great, but I've got to go with the one that introduced us to the world of Panem and the amazing Katniss Everdeen.