I'll go out on a limb and say that it was the end of Season 2; after this, it seemed that the magic and wonder died, and everything that came after was just a long road down.
And I think it really jumped back over the shark again with Wrath of Khan, since they started ruining the old Star Trek by destroying the morals of the original series, as well as destroying where they had already boldly gone before, much better; and the moral was vague and preachy rather than profound and hopeful like Space Seed. Rather, I think it was clearly trying to take after "Star Wars" instead of just being what it was-- i.e. they sold out.
And the rest was a slow nightmare ending with Kirk surrendering to Klingons to save a treaty... a long road down that led to Picard and the Wimp Generation.
Anyway that's my take on it, from the intellectual perspective.
And I think it really jumped back over the shark again with Wrath of Khan, since they started ruining the old Star Trek by destroying the morals of the original series, as well as destroying where they had already boldly gone before, much better; and the moral was vague and preachy rather than profound and hopeful like Space Seed. Rather, I think it was clearly trying to take after "Star Wars" instead of just being what it was-- i.e. they sold out.
And the rest was a slow nightmare ending with Kirk surrendering to Klingons to save a treaty... a long road down that led to Picard and the Wimp Generation.
Anyway that's my take on it, from the intellectual perspective.