At face value, 'Balance of Terror' is as straightforward as they come; this is a classic submarine movie, complete with cat-n-mouse and silent running. Enterprise faces off with a Romulan warbird on the edge of the Neutral Zone; it's uncomplicated, but makes for some tense television.
But then there's the sympathetic opponent, less a villain and more a like-minded captain on the opposite team from Kirk (Mark Lenard plays this well, and I can see why they brought him back to play Spock's father; the two don't seem all that far apart). These are two evenly-matched adversaries sizing each other up. This episode focuses on the profound responsibility of being a starship captain, the immense weight of some of these tactical decisions; and it's a great example of the Kirk/Spock/McCoy dynamic as they hash this thing out from all angles.
In the end, it's just a riveting episode from start to finish; the grudges run deep, the conflict's great, and it's one of the more accessible episodes I've seen.
9/10