What exactly were your criteria for TOS? I think this exercise is useful for the serialized style of DS9, but due to the episodic nature of TOS it becomes somewhat pointless. For instance how is a hackneyed racism plot like Battlefield, or a "Nazis in Space" episode, any more representative of the themes or characters of the series than random episodes like The Enemy Within or Charlie X? And how can something like Conscience of the King or Obsession be excluded when they provide valuable insight into Kirk's character, while Operation Annihilate!, which is only notable for establishing Spock's ridiculous inner eye lid and Kirk's trivial dead brother, is included?
And The Infinite Vulcan?!
How did More Tribbles, More Troubles make it onto the list? That episode was just a shameless, albeit animated, rehashing of the last one!
I'm making my way through a number of different series, and while we might all come up with subjective lists of the strongest and best representatives of the series (which is an admirable goal to help people who want to get a feel for it but don't want to sit through episodes like Shore Leave), I think there's something to be said for taking the bad with the good. That's certainly what I've been doing, and the worse episodes really make the good ones stand out.
My criteria was episodes I found enjoyable, fun and necessary to get a grasp of the kind of show Star Trek was. Cringe worthy episodes like Charlie X, Miri, Catspaw, Spock's Brain, Children Shall Lead, Plato's Stepchildren and those like it didn't factor in to my list because they aren't Trek's brightest moments. While episodes like What are little girls made of, Metamorphis, Requiem for Methuselah, Return to Tomorrow have more going for them and I would recommend seeing them they distract from better episodes. There are more episodes I haven't named that have their own charm to them but I didn't feel it necessary to include them as well.
As for Infinite Vulcan it was Walter Koenig's only contribution to the animated series. That and it's a fun episode. Although the Eugenics War and giant thing was a bit of a stretch.
More Tribbles, More Troubles
I added because it's a follow up to Trouble With Tribbles and if the events of Trial's and Tribblations is taken as full canon then it makes Worf's statements about the Klingon destruction of tribbles more tangible in the sense that the great Klingon Empire did pursue the extermination of tribbles.