After seeing all Star wars movies altogether (plus making of and other facts) it is clear to me now how this phenomenon evolved. Episode IV, in 1977, was made according to older regimes. A hero who saves a princess from a bad guy, with a sci-fi context. The movie is naive, funny and not-so-well-done. Lucas himself admitted that it was a miracle putting this movie together. The major issue was the lack of money, since Lucas wanted to make the movie independent of Hollywood (and he did; all 6 of them). Part of the reason to success was the dynamic at which space was presented; imagine that the best sci-fi movie up to that position was 2001, a completely static and dull depiction of space.
When Episode IV turned out to be a huge success, Episode V had twice as much budget, better production, a lot better special effects, a solid story and the best direction of all 6 episodes (not by Lucas!). The designer of Yoda said he created the face according to his facial characteristics, but then Yoda didn't look much wise. So he took a picture of Einstein and added his cheeks to Yoda to make him look wiser :-)
Return of the Jedi (originally titled Revenge of the Jedi) was a bad movie. Apart from the encounter between the Skywalkers and the emperor, the rest of the stuff is just put together clumsy.
Episodes I and II don't really add anything to the story, although they are artistically orders of magnitude superior to the older trilogy. However the look and feel of the originals is lost: now people are too serious and they are trying to talk big words and they never stop to joke or have fun. The hero-saves-princess theme died with the last Indiana Jones movie.
When all 6 movies are placed equal (their chronological context aside) Episode III is more complete and enjoyable as a movie, followed by Episodes V and IV. If we consider that Episodes IV-V-VI are 20 years older, then Empire Strikes Back is the best film of the series in terms of completeness and coherency in storytelling. Also, watching Episode IV right after Episode III makes so much sense; it's hard to tell that the movies came out so far apart in time.
The most critical character of the whole series is Obi-Wan Kenobi. He is the one who raises and trains Anakin. He is the one who forces him to become Darth Vader by humiliating him. He is the one that Leia seeks in the most desperate of times. He is the one that trains Luke, that finally strikes evil down.
In total, Star Wars seems like the story of Darth Vader. If I have to acknowledge Lucas for one thing though, that is the fact that he created for the first time in cinema history a story that as a whole is greater than the sum of its parts.
