There are so many good books, but the one that always comes to mind when someone asks me what my favorite book is, is The Alchemist by Paulo Coelho. Just read it, its a great story with a good moral.
#2 is the Harry Potter series ... I admit that I'm a sucker for wizards and magic.
#3 is The Help by Kathryn Sockett (SO GOOD!! Just read it!)
#4 is My Sister's Keeper by Jodi Picoult (Way better than the movie, but I cried with both)
#5 The Thief of Always by Clive Barker (Too scared to read his books for adults, but this one isn't too bad. I was slightly scared but it was a fun read)