Every single one except the movies done by Disney. They were- in my personal opinion- major disappointments and full-blow betrayals of the franchise. They did not feel like Star Wars for me whatsoever.
That aside, to answer the thread question, I would say Revenge of the Sith. It had such a great atmosphere and story completion- along with the epic battles and music. The whole transformation of the settings/characters really stood out. You felt emotion in that movie, despite the ever-looming bad acting of Hayden Christensen, lol. It was worth it for the ending.