I actually agree with you that "THE DEVIL IN THE DARK" is the more original story of the two. But "BALANCE OF TERROR" has that suspense and tension throughout that works extremely well, along with a brilliant acting moment from Shatner without even saying a word. (His final walk out if the chapel.)"Devil" is the more original story, taking a cue or two from Twlight Zone about "the baddie that isn't", whereas "Balance" - while excellent - has a few cliches going against it, being elevated more by pace and production tone.
But the last 10 or so on the list are all pretty well-rounded regardless!
Most of TOS season 1 is quite awesome, so I can live with "BALANCE OF TERROR" not making it this year. (It won twice for all of TOS already, and deservedly so.
