I'm throwing my answer in with Rooster, 3,4,5,6 were the best. They complete the entire storyline. Best of luck to you!
I liked the originals but part 3 was good as it explained a lot and you know how and where Darth Vader came about and the beginning of the Empire. Parts 1 and 2 ? They were lousy. 3,4,5,6 . That's the best line-up!