TOS: Balance of Terror, streamed on Netflix.
Simply one of the best, period.
Stiles always takes a lot of heat for his bigoted attitude toward Spock in this episode, but otherwise he really was an excellent officer. He correctly identified the Romulans as being responsible for the outpost attacks, and was right about their ship having the bird-of-prey painted on the hull. His recommendation to fight the Romulans before they could return home, and to violate the neutral zone if necessary to do so, was correct. When the forward phaser room was short on staff, he volunteered to assist Tomlinson, and stayed at his post despite the coolant leak, still trying to fire the phasers even as he was passing out from the poisonous fumes. He never cracked or panicked under fire like Dave Bailey did. If I were Kirk, I would have given him a medal.
Then there is this simple, beautiful exchange, showing the camraderie and mutual respect that can exist between two enemies:
KIRK: Captain. Standing by to beam your survivors aboard our ship. Prepare to abandon your vessel.
COMMANDER: No. No, that is not our way. I regret that we meet in this way. You and I are of a kind. In a different reality, I could have called you friend.
KIRK: What purpose will it serve to die?
COMMANDER: We are creatures of duty, Captain. I have lived my life by it. Just one more duty to perform.