There are many reasons why "City" remains the best episode in my mind. First, the story, at the time, was unique and compelling. Second, the production of it was, for TOS, lavish. They spent nearly $40,000 over budget on it--from $192K to $232K--and it showed. It showed in the sheer number of scenes. Someday, watch it again and count the setups. Far more than any other episode. Joe Peveny worked that ep. It also by showed in going on location. And yes it showed by giving speaking lines to several character actors--the cop, the rodent, Bart LaRue, and even Uhura, Scotty, and Sulu, who at that time were just an additional cost. In fact, I'm trying to recall a first season episode where all of those guys got a speaking part in the same ep. Finally, although it has been criticized by Ellison for the Roddenberry touches (Edith's speech, and more importantly Kirk and Spock's reactions), that bit of bonk-bonk always spoke to me. Gifted insight? Maybe not. But how can you not like the last 3 minutes?