Revenge of the Sith was really good for me at first, and then I realized that it was overly ambitious, with some wonky pacing and terrible acting by a couple people (Hayden comes to mind). I enjoyed it, and still do, but things like "FROM MY POINT OF VIEW THE JEDI ARE EVIL!!!" and I mean, really, most of Hayden's lines, are just bad.
I've gotta cast my vote for Return of the Jedi (even though I voted for Episode 4 in the poll, I just changed my mind). Even with the Ewoks and some of the stupid parts in the beginning (weird alien lips), the movie itself had the most satisfying finale out of any of them. When I saw the big plot twist in Episode 5, it did nothing for me. It wasn't emotional, and it wasn't delivered well. But seeing Luke letting his anger go, cutting off his father's hand, and almost turning to the dark side was one of the greatest moments in the entire series. And then Vader's death and his ultimate redemption complimented it perfectly.