In order, the following are my picks for best fictional book series of all time (that I’ve read):

J.R.R. Tolkien’s The Lord of the Rings Trilogy
J.K. Rowling’s Harry Potter
Orson Scott Card’s Ender’s Game & its parallel series Ender’s Shadow
C.S. Lewis’s The Chronicles of Narnia
Scott Adams’s The Hitchiker’s Guide to the Galaxy
Frank Herbert’s Dune
C.S. Lewis’s Space Trilogy (Out of the Silent Planet, etc.)
Isaac Asimov’s Foundation
Robert Jordan’s Wheel of Time

If interested, please list your own favorites in the comments — especially if you’ve got favorites I’ve neglected to mention or haven’t read myself.