Many people now recommend that newcomers watch the films in this order: 4, 5, (1), 2, 3, 6. For someone coming to the story unspoiled, it flows the best and works really well from a story-telling standpoint to 'flashback' to the prequel films to explain the dramatic reveal at the end of episode 5 and set up the resolution of episode 6. Episode 1 is in parentheses because it is optional--it adds almost nothing to the overall story that isn't established in the others, and arguably weakens it, so some people suggest skipping it altogether.