After read the above defences for this particular episode, I still have to say it was the worst of the lot. It reminds me of when Nelix lost his lungs in Voyager and the Doc sets him up with temp holographic ones whilst there's a frantic search for the originals, another crazy idea.
In terms of
ideas "Spock's Brain", "And the Children Shall Lead", and "The Alternative Factor" were not that bad--it just how those ideas were presented that that made them horrid.
As for ideas that were bad to begin with "The Mark of Gideon", "That Which Survives", and "The Way to Eden" come to mind.
I should say even by TOS standards "Spock's Brain" has a lot of internal illogic. The civilization has ion power-which is beyond Federation technology and in addition they have two other inhabitable planets right in their own solar system. So they don't use their ships to colonize as their planet turns into an ice cube.
Then you have the idea of the men only living on the surface and the women underground. How did that happen? Better yet why is it maintained? Then you have the issue of men becoming primitives and the women idiots--again why?
At least "Spock's Brain" is enjoyable in a Plan 9 sort of way. "And the Children Shall Lead" and "The Alternative Factor" are boring and the less said about the ideas behind "The Mark of Gideon", "That Which Survives", and "The Way to Eden" the better.