Isn't the Leia reveal not until 6? That would be one reason to watch 6 before 3. (if I'm remembering right, it's hinted at in 5 when Luke leaves Dagobah - "No, there is another" - but not revealed until Luke visits Yoda again in Jedi).
Someone said this before on these forums, and I've been enormously happy with this arrangement of films since then. The best order is:
A New Hope
Empire Strikes back
Return of the Jedi
Raiders of the Lost Ark
Temple of Doom
Last Crusade
I don't think it's possible for me to agree with someone more than I agree with this. We watched them and she really liked it, except of course for jar Jar Binks.
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Adults probably already know the Vader reveal, but kids? Kids should always watch the OT first, because:
http://www.youtube.com/watch?v=ZbV5hn_ET0U
And yet, I'm willing to wager 100 internet points that you spent a few moments wondering how to improve upon that list.
Easy... Because Temple is before Raiders chronologically.
http://lexiconmegatherium.tumblr.com/