"Nightmare at 20,000 feet" and "Time Enough at Last" are indeed both brilliant. Nightmare had a young Bill Shatner in the role of the guy on the plane seeing the gremlin on the wing, and Time Enough had a pre-Penguin Burgess Marideth (sp?) as the poor myopic bank teller who looks forward to being the last man alive after armageddon so he can catch up on his reading, only to step on and break his glasses in the final minutes of the episode.
My personal favorites are:
"A Good Life" (also remade for the movie version), with a very young Billy Mumy as the kid with the freaky reality-altering powers who will put you into the corn field if you make him mad
"To Serve Man" - outdone perhaps only by the Simpson's Treehouse of Terror sendup version ("There's still some more spacedust on the book...")
"Eye of the Beholder"
The one where (I don't know the episode name) you think the world is hurtling toward the sun all episode, and in the end the lead woman protagonist turns out to be unconscious wiyh a fever and actually the world is hurtling away form the sun and everything is freezing.
And then the last episode of the series, in the last year when they had gone to an hour format. It was a great adaptation of Ambrose Bearce' (sp?) "Occurence at Owl Creek Bridge" that Sterling had nothing to do, but bought after he saw it, taped himself doing an intro and outro and called it a TZ.
Probably leaving out a couple of really good ones, but those are the ones that left an impression.