Up until now the choices have been rather easy for the most part, but this one is tougher.
Firstly "The Survivor" features one of the coolest looking aliens ever seen in Star Trek,
a Vendorian. Add to that it's not a bad story and we finally get to see a female security officer, Lt. Ann Nored. The part that threw me out of the story was the bit about the Vendorian assuming the form of the ship's deflector shield.
I couldn't swallow that one even when I was 14 years old. Now if he had assumed the form of some of the shields damaged components then I might have been able to swallow that a little more easily...maybe.
"One Of Our Planets Is Missing" is something like "The Doomsday Machine" crossed with Fantastic Voyage.
It's also a decent story even though it feels somewhat derivative. But it certainly feels like something that could have been done live-action if TOS had had access to slightly better f/x resources. However it, too, has its own
moment when Spock reaches out to communicate with the mind of this thing and it turns out to be intelligent enough to understand him. This is where the episode felt somewhat more like "The Devil In The Dark." It's not totally off-the-wall in the context of Star Trek, but it still strikes me as rather out there.
All that said I think I'll have to give the nod to "One Of Our Planets Is Missing" because it's WTF moment isn't quite as WTF!
as "The Survivor."