Here is one order that I would find very interesting to show a new SW fan, and I think they would have to be older to really understand the details of the 6 movies:

1. Episode IV
2. Episode V
3. Episode VI (Then stop it after ObiWan/Luke talk on Dagobah about Anakin.)
4. Episode I
5. Episode II
6. Episode III
7. Episode VI (After Luke leaves Dagobah and the 2nd act of the movie begins)

The reason I would split up ROTJ is it is essentially telling 2 stories that relate to each trilogy. The first 40 minutes are about saving Han, so you are in OT mode after watching Episode IV & V and you are concentrating on those characters. Then popping in the PT after Luke/Obiwan talk can be a huge flashback almost like Obiwan is finally telling Luke EVERYTHING that happened up to his birth, including his mother. Then after that, the viewer is in PT mode, so they can follow the Luke/Vader/Emperor throne room scenes which relate more to the PT, and the rebels trying to defeat the Empire which relates to the overall story of good vs evil.

Just a thought......