I'd go with something more "true to the nature" of Trek which isn't about action or anything like that. But then not picking an episode that does this but isn't too preachy (like Who Watches the Watchers.) I'd also say a S3-S5 episode would make the best selection as those offer the best TNG has to offer without being weighed down by the "growing pains" of the first two seasons or when things started watering down for various reasons starting in S6.
An episode that's not too heavily character-driven or reliant on needing to know how the characters are "supposed to act" or interact or their history. You essentially want the person to be able to sit down and "take-off running", so to speak and to maybe return later to these other episodes that don't make for great first-timer episodes.
Off the top of my head I'm thinking "Booby Trap" as it has a lot of good elements in it and isn't too reliant on needing to know anything about the characters. (Geordi's lack of luck in love is established in the cold open.)
"The Enemy" MIGHT be good but one could argue you have to know and understand the tension between the Romulans and the Federation in order to fully appreciate moments of it, in particular Worf's personal feelings towards Romulans.