At the moment, A Song of Ice and Fire by George R.R. Martin, hands down. The characters are really engaging and it's very hard to put down, once you get into it. The show on HBO is a really good adaptation, too.
Harry Potter. No matter how old I get, this will always be my favorite series. I won't go into it, but simply put, Harry Potter is my world. Nothing even comes to a close second. I'm a proud member of the Harry Potter Generation.