TWOK and TSFS are each like really good TOS episodes. Parts of TWOK are like Balance Of Terror
distilled to perfection, while TSFS has got shades of things like The Trouble With Tribbles
(with it's lighter touches) or even some of the mythological underpinings that came to the fore in something like Amok Time
TFF is like a third season TOS episode after all the money has run out already.