My choice would be "The Enemy" Where Geordi goes blind & is forced to survive on a hazardous planet with the help of a sick Romulan. It's a perfect warm up for a newbie
"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.
"The Defector" would be a much better Romulan episode for a newcomer to Trek.
It goes more into the background of the Romulans, and it is also a great thriller with some good payoff at the end of the episode.