TMP was an episode twice over. First as "The Changeling" and then the aborted "In Thy Image".
TWOK, I could see as an episode if TOS did sequels. But, they didn't and TWOK Kirk is much further along in his life and career than TOS Kirk and it's integral to the plot. So, not really an episode. It's a film.
TSFS -- Spock is brought back to life but not quite all there, the Enterprise is blown up, Kirk's son is killed. Not stuff you'd see in the show.
TVH could work as an episode if you cut out the 23rd Century frame-work.
TFF plays like third season TOS with a bigger budget.
I can't see TUC as an episode but it's a hodge-podge of elements that would make up TOS episodes. Kirk is sent as an olive branch to the Klingons and it goes wrong? That could work. Kirk/McCoy are captured and they have to do a jail break? That's a TOS episode. Investigating who committed assassination during a diplomatic mission and then investigating who killed the assassins? That could be a TOS episode but it wouldn't be someone so close to Spock. It probably wouldn't even be a crew member unless something had gone horribly wrong with them. So, TUC has the parts but its sum adds up to an Epilogue to The Genesis Trilogy not an actual episode from the '60s.
It's really down to TMP, TVH, and TFF.
I'll have to go with TMP. Saving the Whales is more modern than mid-century and the set-up of searching for God only to find out its no God doesn't add up to enough. A probe that wants to become more and join with its Creator is vintage TOS through and through even with the updated and expanded production values.