The creator of Star Wars has stated that the correct viewing order for the original trilogy and prequel trilogy is the numbered order, not the release order, and that watching the films in any other order will result in you failing to get the full impact of the narrative experience that he intended.
"What you've got to remember is that this is one movie. And it's meant
to be watched one through six. So I think when you watch the actual
movie in order, the story will become very clear."
Featurette: The Chosen One
and
"When you see the films 1-6, it's a very different experience then when you watch the films 4 through 6, then 1 through to 3, but the idea that when Darth Vader comes on in Episode 4 it suddenly has a huge powerful effect because you know that, right from the get-go that that Prince Leia's father. You know that when you cut down to Luke that that's his son and you realise that Darth Vader is this pathetic character, he's not this big, all-powerful monster. He's actually a pathetic man who made some wrong choices, who found himself trapped in the world of evil, that he made a bargain with the Devil and now he living in Hell, and the only people who can get him out are his kids"