1. The Empire Strikes Back - Probably the most focussed of all the SW movies. It knows where it comes from and it knows exactly where it's headed. The rise of the Dark Side is extremely well realized and beautifully showcased, among other things, in the Imperial March. I think it's a shame Lucas didn't take this as an incentive to let other do the directing from here on.
2. A New Hope - Simply great fun and a lovely setup for the whole saga. This one benefits from the likeable characters and the great exchanges between some of them. It's also got an epic feel to it which is in no small part due to the wonderful music IMHO. All of the major themes are already here and ready to be used effectively later on.
3. Return of the Jedi - Yeah, the Ewoks kind of suck, and the ending is somewhat over the top (especially with all the shiny, happy Jedi all reunited at the end). Still, it's fun, it's epic (in parts, anway), and it's got all the great characters.
4. Attack of the Clones - Interestingly, again, I find that the middle movie is the strongest one in the newer trilogy. It's far from perfect (especially Obi Wan's unconvincing appearance at the clone factory as well as the Jedi council) but IMHO has a lot going in its favor. In my mind, this is the only one of the prequels where the fun SW banter between characters comes into play. For the first and IMHO last time Anakin and Obi Wan actually click in some way, and it's fun to watch them tease each other. Some of the battles were also great, I thought, capably showing the scale of what was going on.
5. The Phantom Menace - This one would really benefit from a few cuts. It's got some pretty strong elements, I think, such as the portrayel of the old Republic and Palpatine (you can really feel and see why this Republic is just about to fall and why Palpatine really has such a easy time making it happen). I also really liked Darth Maul, and I think it's a shame they dropped him here. I think he should have remained at least until the next movie. What should be cut here IMHO is lots of Jar Jar (though he doesn't bother me as much as others) as well as the whole racing sequence which I just find to be incredibly boring. I also think the film should have spent less time in general on the young Anakin.
6. Revenge of the Sith - This one was a major dissapointment to me. A huge, huge editorial mess IMHO. At this point, developments should just flow but they don't. Instead, you have things like the newly born Vader cry out in anger and pain at the death of Padme, and next thing he's standing on the Star Destroyer's bridge like nothing happened. I also loathed what they did to Padme with a vengeance. She was a strong, independent character in the first two movies. And now? She can't wait to get home and start painting the kid's room. Yeah, right. Also, this one really, really undermines the feeling that the Jedi council was just a bunch of sorry, over-sensitive snobs who basically did everything to further their own demise. The only thing I found really, REALLY powerful in this movie was when the clone army turned on the Jedi. That was both believable (they were, after all, 'just following orders') as well as very well filmed IMHO.
Honestly, I'm not quite sure about numbers 5 and 6 on my list. They may belong the other way around. It's been a while since I've seen those two movies.