I'd rank "Star Wars: Episode III: Revenge of the Sith" 4th out of six. "Empire" still reigns supreme, "A New Hope" works well, and "Jedi" actually has more meaning to me after seeing "Sith". This one was better than the other prequels, but it felt like it had a heavy burden to both work its way out of the mess of the first two films while setting up everything for Episode IV. Parts were really slow or silly, and it wasn't until the second half where the film felt like it was hitting its mark, and then it all fell apart again in the last fifteen minutes, an ending that existed just to give continuity.
Some of the special effects looked bad. The giant iguana that Obi-Wan rode felt unreal, and I thought the space battles were just too busy. You got a big feeling for the scale of society in the film, but that seemed out-of-place compared to the small set of characters that seemed to be involved in everything. I liked the battles between Obi-Wan and Darth Vader and between Yoda and Palpatine, and I thought there was good tension in the scene with Mace, Palpatine, and Anakin, but almost all of the Anakin/Padme stuff was horrible.
If you're a Star Wars fan, you should see the film; it does succeed in defining the history of the Empire, and it gives a good context for the later films . As a film on its own, it didn't work very well.
One joke I thought of on the way out of the theater: "I came here to see a Star Wars film, and all I got was Burning Man!".