The Enemy Within
and What Are Little Girls Made of?
, I think congenially elucidates what Star Trek
is all about and also how dispersive they're both thematically and formatically.
Devil in the Dark
, Dagger of the Mind
& The Changeling
IMO also epitomizes the core of Star Tek
I think neither Amok Time
, Naked Time
nor Mirror Mirror
would be good episodes to start out with unfortunately. As second or third episodes they would of course like any episode be selectable, depending on how you've organized his introduction.