Having freshly watched them, I'd have to say, worst to best:
6. I Phantom Menace. Disneyesque crap--especially the pod racing and JarJar.
5. VI Return of the Jedi. Muppets--nuff said
4. III Revenge of the Sith. How many times did I want to smack Hayden Christensen?
3. II Attack of the Clones. Put me to sleep, but wasn't offensive.
2. IV A New Hope. Was my first "best movie ever." (now Blade Runner)
1. V Empire Strikes Back. Managed to overcome sequelitis, and then some.
I've been pretty outspoken in the past in terms of how I feel about Lucas. He pulls episode IV out of his butt and becomes an overnight legend. He then interviews a million times and starts talking about how IV was the start of the middle act of the real story, and puts out V and VI. Obviously V is good, and VI suffers from "trying too hard" and muppet silliness, but the franchise is in decent shape.
Concerned that the franchise will grow stagnant, he decides to turn the "back story" (aka the first act) into a series, and apparently still has cute on the brain, thus killing the whole franchise dead--but this is Star Wars, so he has at least one more failure coming to him. He releases II, and at least it wasn't a Jim Henson flick, so he gets a third, which at this point is more of an obligation, both for him and for die hard fans.
Sad, really. Apparently, the back story wasn't as developed as he wanted people to believe, and as a result, it had to be "reinvented" in that sad sort of way you see some rock bands today trying to reinvent themselves, and they look like a band trying to be them. In the same way, the prequels tried to be StarWars, and looked silly.
Still, the first couple were innovative in so many ways, not the least of which is the integration of myth into science fiction, by way of The Force. Look deeper into The Force, and you see pantheism, or the nature of government, or both. Importantly, he nurtures the myth, at least from a western standpoint, in showing how the force is used for good, and how the dark side and evil work together. Sadly, by the time Lucas gets to writing Episode I, he's writing childrens' films, and seemingly forgotten what's so great about myth--at one point entirely attempting to reconstruct it scientifically by inventing "midi-chlorians."
So it is, we saw an unbelievable spectacle in the original film, and watched him outdo himself in the second, before complacency came in, and he Jim Henson'd the enterprise. We forgave him, though, and respected the franchise enough that when he came rolling around for more money pitching the prequels, we went to see them, almost out of duty.
I wonder what it's like to do your three best films at the very beginning of your career and then come up empty. At least he has the tremendous businesses he built around the franchise.