Reading some of these posts makes me feel old, considering I saw the original Star Wars in the theater when it came out in 1977. Of course, I was only 4 at the time, but I still remember it quite clearly.

If I had to list the series from the best to the "worst" I would say this:
Episode IV: A New Hope
Episode V: The Empire Strikes Back
Episode VI: Return of the Jedi
Episode III: Revenge of the Sith
Episode II: Attack of the Clones
Episode I: The Phantom Menace

The original trilogy, though, was hard to put one over the other. All three of them were equally good, so I simply put them in order of their appearance.