@kev I voted for release date order but here's what I really think. I would watch Episodes 4-6 first, which matches release date order. Then, I'd watch the rest of the movies in chronological order.
I have 4-6 first only because they are older. They'll look even worse if you watch the newer ones first. Also, you get a nice intro into the original concept by George Lucas first before getting into all of the expanded stuff.