Though definitely one of my favorite Season One episodes, I find myself unable to vote for "Balance of Terror" because it's too derivative, IMHO. As an analogy to The Enemy Below
and other WWII themes it's fantastic, but Star Trek wasn't just about the visualization of analogies in a futuristic concept.
My choice is the first episode produced for the regular series, "The Corbomite Maneuver"
- A "first" (pun intended) to set the style and tone for the series without the need to conduct additional "soul searching" as we've seen in later spin-offs
- delivers "Where No Man Has Gone Before" right from the start by carrying us into the unknown and uncharted regions of space
- shows us how to deal with an overwhelming adversary - by using brains and imagination
- great morale that "appearances can be deceiving"