It occurs to me, strangely enough, that I think like this:
VI / V > IV / III > II > I
With VI and V being a bit like LTTP and LA to me- one's the more favorite, but I really love them almost equally; it's just a few certain factors tipping one to be my favorite. In all fairness, V comes off as being more serious at some points (the ewoks, while sort of cute in a weird way, are also horribly silly), but the big climax of VI- Luke/Vader/Palpatine's scenes, and the rest of the ending from there- were really wonderful, and render it my favorite.
IV is a wonderful movie, beautifully assembled for the circumstances in which it was made and the time it was made. It gets the most originality points, of course; but it seems a little less deep in retrospect, because less is going on. It's more about something good starting, "A New Hope" for the ruin left behind by III.
III, on the other hand, is a damn good movie, made as well as possible for the present, far exceeding I and II. Pretty powerful acting, a deep and emotional story; they don't go into overdetail on things, and the plot is pretty clear-cut (as well as being something we could all pretty much figure out after having seen the other five), but that's not really a bad thing. II's politics can be a little hard to follow if you can't pay really, really good attention to everything and really get a grasp on it. I can find no real bothersome faults in III, and do not care to invent any to complain about; it was well-made and tied the series together quite well.
II was reasonably good, but not really awe-inspiring; but it was definately an improvement over episode I. The action seemed overboard, and the plot seemed a little choppy-paced, but overall it was a pretty good movie. Like a four-star movie but not quite a five star, so to speak.
I... bleh. It had good traits, and for coming out so much later it did a decent job of picking up on some of the same traits, but the movie feels silly and mediocre. Senseless podracing hijinks and a horrible race (and main character) spouting dialogue in voices offensive to human ears throughout. Liam Neeson was cool here, though; I found myself mostly paying attention to Qui Gon and Obi Wan in this movie. *shrugs* Really kinda on the three star end...
All in all... Episode III was by far the best of the prequels. Worthy of the original trilogy. But not at all a kids movie, people below the age of 12 will probably have nightmares from it and might start bawling during the movie when "the hero" starts chopping up children or his skin burns off or something. I wouldn't call I or II really revolutionary, but this movie is very good indeed.