The original Star Wars was great. It's my favorite movie. The Empire Strikes Back was awesome too. But then, the Ewoks ruined the whole taco.
My nod goes to the Lord of the Rings. It got better with each movie. The Return of the King was the best. Expectations were met and exceeded beyond my wildest dreams. I read the books many times and always wondered if it could be made into a film that captured the scope. And then the vision was realized fantastically.
Bottom line- Peter Jackson was obsessed with making a great body of work. Somewhere along the line, Lucas got obsessed with making money and the art suffered.