Before the release of Revenge of the Sith I was pondering which movie would be tops once everything was done. In the past, I, like many others, have tended to lean toward Empire Strikes Back as my top choice. Now that all the movies are out and I've watched them all repeatedly (yup, fanboy), I realize that I can't really pick one. By and large, they all have something to recommend them. Unfortunately, I do have a least favorite -- Attack of the Clones. But otherwise, there's something to recommend each movie, and perhaps things that detract, as described in the extended.
Episode I - The Phantom Menace
Pluses: Starts with Jedi in true form. Fine acting from Liam Neeson and Ewan McGregor. The concluding lightsaber fight is not just a good action sequence, but also a good psychological comparison between balance and rage. The Pod Race is honestly pretty cool.
Minuses: Wooden acting from Natalie Portman (which is okay when she's in "Queen" mode, but is weird otherwise). Too many endings. Anakin goes from cool-and-collected in the Pod Race sequence to little-kid-accidentally-killing-ship in the final battle. Jar Jar is a little much.
Episode II - Attack of the Clones
Pluses: Several good action sequences. Yoda fighting. Clone gunships. Coruscant city life. Again, Ewan producing solid work.
Minuses: High-school-play acting in the Padme/Anakin love story. Hayden whining just a bit too much. Poor development on the love story.
Episode III - Revenge of the Sith
Pluses: Excellent action sequences. Tremendous performance from Ian McDiarmid. Sweeping political action and betrayal. Epic showdowns.
Minuses: Hayden still a little whiny, Natalie still kinda wooden.
Episode IV - A New Hope
Pluses: Stellar opening. Witty dialogue with generally good delivery. Another good showdown reflecting the psychological differences between Jedi and Sith.
Minuses: Fighter combat hasn't aged well (quite slow compared with all the other movies). Fairly simplistic plot.
Episode V - The Empire Strikes Back
Pluses: Epic opening battle that the good guys lose. Solid dialogue and acting. Great philosophical discussions. Beautiful showdown between Vader and Luke. Downbeat ending to set up the following "act" (movie).
Minuses: Of all the movies, I find Empire the most "minus free."
Episode VI - Return of the Jedi
Pluses: Fun opening sequence. Great final space battle. Wonderful showdown between good and evil where good wins by refusing to fight.
Minuses: Ewoks slaughtering "an entire legion of my best troops" on Endor.
Overall, I like the OT for its snappier dialogue and plotting, but I like the PT for its broader scope and political story. Based on what I've written above, I might rank them:
1. The Empire Strikes Back
2. Revenge of the Sith
3. Return of the Jedi
4. A New Hope
5. The Phantom Menace
6. Attack of the Clones
But slots 2 and 3 are pretty interchangeable.