4. "To Serve Man"
Season 3, Episode 24
This is the best example of a strong half hour being completely overshadowed by a twist. Before discovering that the alien Kanamit book "To Serve Man" should have "with Hollandaise Sauce" after it, the episode questions whether an outside force -- alien or human -- can be trusted to have pure motivations. The ending, where our hero sits on a spaceship, resigned to his fate as a future main course, is bleak, but what’s most insane about the episode is that the codebreakers who initially translate the title of the alien’s book are too credulous to decode what's beneath the cover.
3. "Five Characters in Search of an Exit"
Season 3, Episode 14
A soldier, a ballerina, a bagpiper, a clown, and a hobo wake up inside a rounded metal pit and no memory of who they are. No, this is not the beginning of a bad joke, but a riff on Sartre, a puzzle of identity and purpose. A syrupy ending (this was the Christmas episode, after all), it takes nothing away from the episode's absurd terror, which also pokes meta-fun at the series: the possible explanations for the trap (a dream, a dream within a dream, hell, aliens) all recall twists from other episodes.