Star Trek Anniversary - John's Top 20: "Balance of Terror" (#5)
At #5 on my top 20 list is "Balance of Terror." This is an episode about the utter futility of war, and the fact that -- as starship commanders -- Kirk and the Romulan (Mark Lenard) are, perhaps, best equipped to see that; yet bound by their duty to fight.
The story also features a subplot about bigotry, involving a crewman's dislike of Spock.
In terms of franchise importance, this episode introduces the Romulans, the "bird of prey" designation (later given to the Klingons), and the Romulan cloaking device.