OK there are a lot of Romulan episodes on this list (seven) and the one that started them all definitely deserves a place. Not only did it introduce Romulans to Trek, but Mark Lenard as well. While they would not prove as iconic to TOS as the Klingons, the TOS concept of the Romulans would endure through the franchise much better than the TOS Klingons did. The battle of wits between Kirk and “the Romulan Commander” (He is never named.) keep you on the edge of your sofa, with the one crewman’s antipathy toward Romulans/Vulcans (so soon after World War II) the most plausible space racism in TOS.
An unofficial companion piece to BoT, The Defector is the quintessential TNG Romulan episode: treachery, continuity, strategy, a Romulan admiral lecturing Picard about the importance of family, and some great Shakespeare crap. Andreas Katsulas cemented his role as Tomalak with this one, which had the twin-bladed effect of landing him G’Kar on Bab 5 while regrettably taking him away from TNG.
Also Picard owns the crap out of them in this scene here: