I have to go with two. The first is in "Amok Time" when Spock sees Kirk alive and the smile lights up his face. It speaks volumes to the depth of the Kirk/Spock. relationship.
The second is in "The Doomsday Machine" when Spock relieves Decker from command. These is such an intense level of vulcan bad-assery when Spock motions for the security guards to come over.