It is sad that most of the younger ppl now watches it in episode order. The films should be watched in release order. The original trilogy story structure was designed and written in way that you start with Ep4. The prequel trilogy as its name clearly indicates is a back-story to original trilogy so it should be watched second.
There was a big debate about what order the movies should be watched in. A general agreement was 4,5, (optional 1), 2,3,6. Watching 4, 5, then having the prequels kinda work as a flashback, then coming into 6 with the knowledge of the prequels. That way someone meets him as Vader, gets the suprise of "Luke I AM YOUR FATHER!", then finds out about a young Anakin, then you come back and get to see him redeemed by his son. Again, 1 is optional. It's not needed for the story and can be watched afterwords as well just to see Darth Maul and Jinn.