Agreed, so here's my runner up:
End of Act Two of Balance of Terror.
Red alert / battle stations kicks in.
Kirk: "I hope we won't need your services, Bones."
McCoy: "Amen to that. You're taking a big gamble, Jim."
McCoy leaves the briefing room, Kirk follows, but Sulu is holding the door open. Kirk exits and the camera tracks with him and Sulu as they stride the corridor as music from "The Naked Time" plays over the shot of the busy hallway.
It's bursting with energy and urgency and still brings a chill to me. It's a hell of an act-ender.
One more:
"What is it cap'n?"
"Cestus Three as been destroyed." Boom. That's it. No Kirk out, just off to the opening titles.
That's a great one too...100% agree.