Kirk, Spock, McCoy, Scotty, Sulu, Uhura, Chekov, Chapel
What to you is the single greatest moment for each of them in TOS? How you want to define "great" is up to you.
Kirk: Outwitting the Orions in Journey to Babel (you speculate)
Spock: Smiling at Jim in Amok Time
McCoy: Standing up to Khan in Space Seed (carotid artery)
Scotty: Not allowing the Eminiarians to trick him in A Taste of Armageddon
Sulu: Attacking the crewmen in the corridors in The Naked Time
Uhura: Distracting evil Sulu in Mirror Mirror
Chekov: Sitting in the cafeteria with Scotty in Trouble with Tribbles
Chapel: Waking Spock up in Amok Time
What to you is the single greatest moment for each of them in TOS? How you want to define "great" is up to you.
Kirk: Outwitting the Orions in Journey to Babel (you speculate)
Spock: Smiling at Jim in Amok Time
McCoy: Standing up to Khan in Space Seed (carotid artery)
Scotty: Not allowing the Eminiarians to trick him in A Taste of Armageddon
Sulu: Attacking the crewmen in the corridors in The Naked Time
Uhura: Distracting evil Sulu in Mirror Mirror
Chekov: Sitting in the cafeteria with Scotty in Trouble with Tribbles
Chapel: Waking Spock up in Amok Time