Technically, the best order to watch them in is 4,5 (1,2,3) 6 Since the vader reveal is completely ruined by the prequels, you should watch them as if it's Luke's story up until the end of Empire and then it becomes a massive 3 film flash back that covers vader's backstory/Luke's heritage, then you conclude with 6 and the simultaneous end of both Luke and vader's story.