I personally favor the prequels because of their spectacular visuals. They are what got me interested in costume design, concept art and the art of filmmaking. However, the originals are superior for their acting, directing, scripts and story.
My favorite prequel is Revenge of the Sith, and my favorite original film is The Empire Strikes Back. But if I had to choose just one, I would have to say Revenge of the Sith. It is a fantastic climax, trying all the other films together.