1. Season Two
It shouldn’t take a Vulcan to suss out that this would be the number-one season in my ranking. By season two, Star Trek had hit its stride and was regularly putting out good episodes. There’s a certain sense of self-assuredness in the episodes of this season. It’s like everyone involved—from writers to actors to production designers—knew what Star Trek was about and were into it.
The result of this confidence (and quality) is that the episodes freely explore a myriad of concepts and stories. The dynamic of Kirk, Spock, and McCoy as the main trio was solidified by this time, and you can tell the writers (and actors) were reveling in putting the characters in different situations and seeing how they would respond.
As I said, the first and second seasons of TOS are fairly comparable in their quality. The reason season two ranks higher is that it doesn’t have the new show growing pains that season one has. I’m not sure any season two episode reaches quite the highs of “The City on the Edge of Forever,” nor the lows of “And the Children Shall Lead,” but it is solidly good throughout.
Season two provides a good sampling of all the different tones and styles that the original series works in, and all at a fairly good standard of quality. If I could only recommend a single season of Star Trek (any Star Trek), it would be this one. It shows how Star Trek can be both serious and fun. Plus, it introduces Pavel Chekov, tribbles, and Spock’s parents. What’s not to like?