Attack of the Clones, because I feel that the romantic scenes weren't that great, and it's because of the writers. I absolutely adore the romantic aspect of Star Wars, but I don't think the writers made some of the scenes "romantic" enough, partially because they're sci-fi writers and not Jane Austen. So, that being said, I believe that if they focused less on the romance in AotC, the movie would have probably had better lines and such.
I didn't like "Attack of the Clones" as much as the other two prequels. The dialogue between Anakin and Padme in the romantic scenes was strange and it wasn't Natalie or Hayden's fault. The script should have been written better for them. Other than that reason, the movie was good especially the Geonosis scenes. I liked Episode IV, V, and VI equally.
"RETURN OF THE JEDI". I thought it lacked originality, aside from the final scene aboard the Death Star. And although I am aware that the acting in the six STAR WARS movies tend to be cheesy, the acting by Carrie Fisher and Harrison Ford just made it even worse in this film.