Star Wars (1977) possessed an energy that none of the sequels/prequels have been able to match. It has a certain freshness and innocence to it. The terrible shooting conditions, time constraints, and limited $ and technology forced talented people to great heights of innovation and creativity. It also has the best pacing of any movie I have ever seen; a lot of the credit goes to the editing skills of the two Lucases (Luci?).
Empire, for all it's merits, is a Hollywood film. Quite frankly, I do find Empire a little boring at times and I sometimes think the "darkness" of the material actually did more harm than good to the series.
Return, for all it's faults, is Hamill's best performance. Since he is what the series is all about I always thought Return was underrated. I can see the critics point of view of course; the totally mailed in/coked out performances by Ford and Fisher, teddy bears etc.
I've become more and more like Anchorhead - the original film is kind of in a separate category of it's own and is almost always the film I choose to watch when I watch a Star Wars movie. It is, in a way, the only Star Wars movie. The truest to George's vision from 1975/76 (no matter what he now claims).
The prequels just suck, although TPM was at least an attempt to make a film with an original vision. I think. It still sucked.