2. Season One
In many ways, the first and second seasons of Star Trek are fairly comparable in quality. The biggest difference is that the first season takes some time to settle. Every version of Star Trek has this issue in its first season, but TOS may actually suffer from this phenomenon the least.
Whereas Star Trek: The Next Generation’s first season has a handful of good episodes amid a sea of episodes ranging from inane to awful, the original series has a really solid slate of episodes. Most of the issues are honestly more about production and continuity than the stories themselves.
It takes a while in this season (especially if you watch in production order) for the series to fully sort out the look of Star Trek. Costumes evolve and props get refined. Similarly, some of the lore is fuzzy early on like early episodes calling Spock a “Vulcanian” and vagueness as to whether the Enterprise just represents Earth or represents the entire Federation.
None of these critiques are really problems, I would say. They are just a bit distracting, especially once things are more firmly established within the continuity of the original series (let alone the continuity of Star Trek more broadly). In some ways, these aspects of the season can be a cool way to see the creative process unfolding before us.
Overall, Season one of Star Trek is really good, with strong writing and characterization. Even some of its earliest (produced) episodes are some of the strongest in the series, and episodes like “Arena,” “Space Seed,” “Balance of Terror,” and “The City on the Edge of Forever” are classics and among the best episodes in all of Star Trek.