If it's overall Trek, it has to be something from the Original Series to get someone into the groove of things. I would say something from the first series so as not to disorientate them too much - you don't want the absolute best episode to lead them to sudden disappointment later on, nor do you wish to subject them to the very worst and put them off. For that reason, I think "The Devil in the Dark" is probably the best one to start off with - it's a story that still holds up very well today. And it's one of my favourite of all the TOS episodes.