In the number 19 slot, I have placed the deeply underrated third season entry, "Spectre of the Gun."
This episode is an almost perfect encapsulation of Star Trek's values.
Specifically: first contact with an alien race is terrifying and deadly, until a crisis is worked through; and humanity showcases some essential quality that makes it, in the eyes of the aliens, noteworthy.
Also, the visual style of this episode evokes the surreal. It looks and feels like a nightmare, featuring the "zombie" Earps and other unique touches.