Star Trek: the Original Series
This is an undisputed classic episode. When I first saw it as a kid, the Planet Killer scared the you-know-what out of me. The suspense in this episode can be cut with a type-1 hand-phaser.
Everything about this episode was effective, from the conflict between Commodore Decker (William Windom) and Spock, to the horror of being drawn inexorably into the gaping maw of the Planet Killer, to the final climax of Captain Kirk waiting to be beamed back to the Enterprise before he is either consumed or blown to pieces. "Gentlemen, I suggest you beam me aboard."
The Pup’s Response
Loved it! This episode was right up there with "Balance of Terror".