TOS - Where No Man Has Gone Before.
Amazing they aired that near the middle of season 1, seemingly at random, despite the less-refined bridge set, different outfits, Spock acting a bit more shouty than usual, new crew out of the blue... (yes, all to save money, as given the money they gambled on the failed 1964 pilot they had to cut corners somewhere. Hence WNMHGB being thrown in as well as "The Cage" being (very cleverly re)used into another episode.
"Where No Man Has Gone before" was the third episode to be broadcast in the USA, and thus far closer to the beginning than to the middle of season 1.