First time viewer 4,5,6,1,2,3.
Other than the quality of the films themselves nothing fails, the surprises in 5 and 6, the effects get better, the inpending doom of 3.
Repeated viewings 1,2,3,4,5,6.
The suprises don't matter, the effects are rarley even thought of and it feels like long journey (in a good way).
My sister was adament about watching them 1-6 because she saw 1 first but it failed miserbly. 3 was the worst of the prequels in her opinion.
When fans watch 3 they see:
X-Wings before they were X-Wings.
Tie Fighters before they were Tie Fighters
Glad they know how the Palpatine became the Emperor he is in 6
Find out how the Jedi's became extinct.
See the Blockade Runner from the start of 4 in new condition.
The huge Duel they have been waiting to see for 20-30 years that made Darth Vader what he is.
See Obi-Wan take Anakin's Lightsaber for Luke.
Find out how Obi-Wan will be able to communicate with Luke after his death.
See the start of the Death Star build.
See where Leia went and her parents that die in 4.
Watch luke get dropped off and watched from a far.
Nearly everything in the film is a wink or nod to things you wanted to know/see.
When first time viewers see it they see Sam Jackson die, a background character become wrinkly and in charge of Anakin. Anakin get REALLY hurt and some big mechano kit being built at the end. It means very little.
When my sister watched 4 she thought it was OK but wanted to know why Darth Vader never realized he was threatening his Daughter and why the Lightsaber fight looked like two old women fighting, when we watched 5 we did it in two sittings because she was that bored.
Number 5 works when you:
Don't know who Yoda is, he's mystical.
When Obi-Wan say's "He's our last hope" and Yoda say's "No, there is another. Your dying to know what they mean.
Don't realise that Luke only does about 1% of normal Jedi training to become a Jedi Knight.
You don't know who Lukes Father is.
The big reveals in 6 are totally ruined, Brother and Sister, Darth Vaders mask coming off.