Yesteryear
Maybe I'm a little biased since I just saw Enterprise's Forge Trilogy and Journey to Babel recently, but I loved this episode. It made me question why this series wasn't canon because we learned so much about Vulcan here that was only heightened by those 4 other episodes.
Well, Roddenberry issued a memo in, I think, '89 in which he asserted
his position that TAS wasn't part of the canon, a position he took because it had some rather fanciful elements here and there, and also because he wasn't directly responsible for producing it, serving in more of a consultant capacity. Generally, his view in his later years was that any ST he didn't produce was of questionable canonicity at best; he considered ST V non-canonical as well.
However, contrary to popular belief, that memo ceased to be binding the day Roddenberry died. Canon isn't some law handed down from on high that all subsequent creators have to obey, it's just the policy and preferences of whoever's currently making the show. And a lot of ST productions in the years following Roddenberry's death have included references to elements of the animated series, such as DS9 mentioning the
Klothos as Kor's ship and ENT using Vulcan's Forge. And of course the scenes of Spock's childhood in the 2009 movie are pretty much a paraphrase of a couple of scenes from "Yesteryear."
So I think the "TAS isn't canon" idea can be safely regarded as a dead letter by this point.
We finally got to see Archeia
Wha? Oh, you mean I-Chaya.
(and I'll say the Sehlot's in Enterprise were a lot more menacing, this one really was a teddy bear

)
Well, I-Chaya was old and weak, not to mention domesticated.
And note how unusual it was for the time to tell a story involving death in a Saturday morning cartoon. Filmation would return to this well two years later in the
Isis episode "Lucky," in which Isis helped a young boy deal with the (offscreen) death of his beloved dog. I remember being quite affected by "Lucky" when I first saw it as a kid.
"One of Our Planets is Missing" is enjoyable (even if it's just the Space Amoeba with intelligence).
I've always seen it more as a precursor to
Star Trek: The Motion Picture. A vast cosmic cloud is heading toward a populated planet, the
Enterprise passes through its defenses to its brain center, Spock melds with its consciousness and the heroes try to persuade it that the little mites occupying its target planet are intelligent beings.
I suppose it could also be likened to the Galactus Trilogy without the Silver Surfer.
Although why Spock and Keniclius had to be 50 feet tall eludes me.
Because they could be. The goal was to depict things that couldn't be shown in live action. I guess normal-sized clones weren't visually distinctive enough.
Sure, Spock convinced the cloud not to ingest Mantillies and to go back where it came from, but wasn't the Cloud's purpose seeking out food to survive. Maybe I'm not understanding it, but couldn't a compromise be reached to save Mantillies and still allow the cloud to ingest food?
There are a lot more uninhabited planetary bodies in the galaxy than inhabited ones. The cloud should be able to feed itself quite adequately while still choosing to avoid populated planets.