There is no doubt in my mind that this is the best episode of the Star Trek franchise - that includes the other Star Trek series that followed in the 80s and beyond.
"City on the Edge of Forever" combines so many wonderful and riveting elements - time travel; the fate of not only Earth on the line - but the universe; and a heart-wrenching love story involving a choice that no one should ever have to make, that between saving someone you love versus saving billions and billions of people.
"City" is crackerjack from the start, with the first quarter of the episode on the ship and a planet having a strange ancient, abandoned civilization - as well as the mysterious time portal called the Guardian of Forever.
It has a terrifying and unique "Captain's Log" entry from Kirk, i.e., "Captain's log - NO star date - for us, time does not exist". All history has been changed by Dr McCoy, who has gone back in time to 1930s Earth.
By the time Kirk and Spock miraculously track him down in the past, Kirk has fallen head over heels for a woman, Edith Keeler, and is ready to live his life in the past, with her, until Spock finds out that Edith must die to set the "future" straight.
It all leads to a horrific traffic accident in which she dies - an accident that Kirk could have prevented if not for Spock's haunting words, earlier in the episode, "Do as your heart says, and millions of people..." who were never meant to die, will do so.
Shatner, Nimoy and Kelley are top-notch, as is Joan Collins. Yes, THAT Joan Collins, from "Dynasty" and an assortment of B-movies. She's utterly wonderful, despite her British accent being out of place in 1930s New York City.
Some Trek episodes come close to the greatness of "City", but fans of the original Trek know that "City" is an episode that they can watch and cry over, over and over.
As Kirk says at the end of the episode, once "all time is restored" ... "let's get the hell out of here...".