It always blows my mind when I hear about someone who hasn't seen the Star Wars movie.
Really, there's not really a good reason to watch them in any order other than IV, V, VI and then if you really feel the need to watch the prequels, continue with I, II, III.
Keep in mind, the prequels feel very much different and that George Lucas had already lost his mind by that point.
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"Filthy lawbreaker, unhand that vending machine!" - Casey Jones