Start with ANH, definitely. If they just want to see Star Wars, then they can stop after ANH, since that movie was originally just called 'Star Wars', it tells a perfectly fine self-contained story, and with any luck, it will interest them in watching ESB and ROTJ. Ta-da, they've seen Star Wars! (The ones that matter, anyway.)
If they want to see 'the whole story', I suggest Machete Order: IV, V, II, III, VI. Yes, there are only 5 movies listed there--Episode I is just a giant lead weight dragging the entire series down. Nobody really wants to see Darth Vader when he was an annoying little rugrat played by the kid from Jingle All The Way. Nobody really wants to see an acting juggernaut like Liam Neeson have to deliver physically painful lines about midi-chlorians. Nobody really wants to watch a movie where the plot is about tax law. If you take out Episode I and show II and III as a duology in between V and VI, then we see the story of Anakin Skywalker the Jedi, and see how he fell to the Dark Side, and then we see Luke defeat Vader and the Emperor, redeem his father, and save the Galaxy from the Empire.
Then if they still want to see Episode I after all of that, you can show it to them, and they'll understand just how awful it is. Or show it to them in the context of other 'kid's stuff' like the comics, some of the novels, the Clone Wars movie, etc. (Though to be fair, the Clone Wars CGI series is pretty good, and runs on both Cartoon Network's daytime programming and on Adult Swim, which should indicate something about the series. If the person you're showing Star Wars to really enjoyed the movies, show them the series. It's way better than Episode I.)