I'd actually show them some videos they would already like that have mlp references in them. That's how I go into they show, by watching mlp youtube poops. But if I had to choose on episode, I'd say "The Return of Harmony." Even though the 1st episode would seem to be the best choice because it sets up the characters, I just don't think its that great an episode. Return of Harmony also has an interesting villain, Discord, and definitely proves that mlp isn't always a girly show. Oh, and the worst episode to show a non-brony would be "Hearts and Hooves Day." That's only the worst episode IMO, but also quite girly, too.