1. Empire Strikes Back. It's just so dark and cold and... haunting. All that snow and swamp. On the bad guy side there are AT walkers, Boba Fett, and Vader taking charge with no Tarkin or Emperor in the way. On the good guy side the characters from the first film get fleshed out and the romanctic stuff actually works.
2. A New Hope. Understandably the most dated-feeling (almost "quaint") due to its age and budget restraints, but that also makes it pure old-fashioned cinematic fun. The fantasy staples of the young hero of destiny, the old wizard, and a princess to be rescued are just so much better on the big screen than watching stiff guys stand around talking about galactic politics.
3. Revenge Of The Sith. Some pretty good payoffs as most of the loose ends get reconciled in a satisfying way. The Emperor's scheming in the middle of the film is terrific. But some of the dialogue is head-shakingly bad, and I can't help but feel that the final opportunity to give a few minutes' screen time to The Man In Black, one of the most memorable characters in cinematic history, was squandered on a cheesy note.
4. Return Of The Jedi. Had this one behind AotC for a while, but Episode III makes the throne room scene, already done very well, even better. Too bad it still has those accursed Ewoks, a lame death for Boba Fett, and a lot of the actors just appearing to mail it in. No wonder Harrison Ford wanted out, Han Solo's character is especially disappointing.
5. Attack Of The Clones. I think this one maybe got a bit overrated when it came out because people were so relieved it didn't suck as bad as TPM. Liked some action scenes such as the Jango vs. Kenobi stuff, but the romance is just plain bad. And looking back, the whole arena scene is a little bit silly. I may yet reswap ROTJ in the #4 slot with this, they're pretty close together in my mind.
6. The Phantom Menace. Let's see.... Jar-Jar, midichlorians, and an absolutely horrible climax to the big space battle. Yeah, Darth Maul was cool but I didn't buy the way he was made to die. Agree with the previous sentiment that a lot of this film didn't feel all that necessary. The core of it could have been done as a few minutes of expository dialogue ("the Force is strong in that apprentice of yours, where did you find him?") and maybe shown in a separate stand-alone kiddie cartoon or something, which would free up a whole movie to cover a better story like the opening years of the Rebel Alliance or something.