I'm going to have to agree with you largely for the same reasons. This is where it all started, it's the reason we're still talking about it, it was fairly progressive (The top most computer expert in the Federation is a black dude?), and there were bonafide science-fiction writers involved. I think "The Alternative Factor" is the only 1st season episode I don't really like but there are bangers like "Errand of Mercy," "City on the Edge of Forever," "Space Seed," and one of my personal favorites, "Balance of Terror." Damn, I need to go watch some 1st season TOS.TOS Season 1. That’s the only correct answer.
I absolutely agree. As a kid, when it first dropped on air, it was absolutely revolutionary TV. The crew looked like a United Nations sit down. There was a combination of Wild West shoot-em-ups, monster movies, WWII submarine warfare, and thought provoking cerebral stories. you're right that later series used elements of TOS stories, to the point of essentially ripping them off entirely. The members of TNG bridge crew are essentially the TOS crew, split into multiple bodies.TOS Season 1. That’s the only correct answer.
Let me explain-
1. Without it, there is no other Star Trek.
2. It introduces almost all of the concepts and terms that form the basis of later Star Trek.
3. The writers are a murderers’ row of talent.
4. There are 29 episodes (!?!) of all killer, no filler. Look, I’m not going to say that every single episode is a stone-cold classic, but even the occasional FINE episode (e27) introduced parallel universes!
5. It contains Harlan Ellison’s City on the Edge of Forever (e28).
I could see arguments that season 2 is, overall, slightly more consistent. But season 1 has to win because it started all of it. And no season of any Star Trek had as high of a classic bangers/misses ratio as the first two seasons of TOS.
Even ignoring the fact that it pioneered the whole “Star Trek” thing, it would be trivial to point out how many plots of later series and episodes are derivative of what was done then.