The Devil in the Dark - 2
Mirror, Mirror - 1
Both are great episodes, but "Devil" suffers from the hokeyness of people getting killed on guard having plenty of time to fire but inexplicably not doing so. Kirk and Spock could have concentrated fire on a spot already hit by a killed guard.
"Mirror, Mirror" has its problems too, such as some arbitrariness with the application of the Tantalus field, but overall is utterly brilliant: what if "the Federation" took dilithium crystals by force. "Devil" is really brilliant too, with the monster that's not really a monster at all, but "Mirror"'s problems just don't bother me as much.