If someone has never seen Star Wars this is the best order to show them.
A New Hope: Give them the impression Luke's father was a hero. Darth Vader murdered him along with the Jedi and ruined everything throwing the galaxy into fascism.
The Phantom Menace: We see the Jedi in their prime, and we see Luke's father as a little kid with so much potential. It's so vibrant you wonder how everything can go wrong.
Attack of the Clones: We see Luke's father as a hero and as a Jedi. We see him fall in love with Padme. We also see the galaxy pushed into war.
Empire Strikes Back: We see Luke continue the struggle against the Empire. He is instructed by Yoda. He faces off against Darth Vader the man who killed his father only to find out he IS his father.
Revenge of the Sith: We find out why Anakin turned to the dark side and how he became Darth Vader.
Return of the Jedi: And now we can see the final confrontation as Luke tried to redeem his father.