Night Call, Five Character's In Search Of An Exit, The Lateness Of The Hour, The Encounter. These are one's that come to mind.

Several great episodes to choose from, but I'd have to lean with "The Changing of the Guard". You can tell the hurt in the old man's eyes when learned he isn't wanted anymore. Great ending, however.

The Long Morrow.
and
Number 12 Looks Just like You.
What if this is as good as it gets?

Ring a Ding Girl was a sad one in my opinion. Bunny Blake seemed to be a spoiled movie star, but when she insisted that her sister skip the picnic her sister said 'alright honey, if it means that much to you, we won't go to the picnic' (paraphrasing). I just really loved that scene. I just liked watching the relationship between the two sisters in that episode; one was still a small-town girl and the other was a big star. Great acting and sad ending in that episode. It's a favorite of mine.

Young Man's Fancy in Season 3. Sad, bizarre tragedy.
Giddy-up, goat!

the big tall wish - great episode

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Mine would have to the be THE HOWLING MAN. That ending still gets me everytime I watch it. The guy let the devil out and so guilty he spends years looking for him and then finally catches him. But at the end where the maid opens the door, u just know what's gonna happen!

The ending where Jack's character surrenders his own life so that his dying son (fighting a War in Asia) can survive.
(`
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).)^

I thought "Ninety Years Without Slumbering" was sad! They were so mean to Ed Wynn's character. I also thought the ending of the ep1354isode of the robotic grandma was sad.

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definitely Time Enough At Last with Burgess Meredith

The one about the guy with the stop watch. I used to wish I had one of those.

"Walking Distance". I weep every time I see it.
"Some wisp of memory, not too important, really. Some laughing ghosts that cross a man's mindthat are a part of The Twilight Zone."

It's already been mentionned but I'll go for "Nothing in the Dark".
Well, the Jerk Store called, and they're running out of you.

The Gift (Season III, Episode 32)
: It's so demoralising that I won't ever be able to watch it againwhat a sad testament to this world.

"On Thursday We Leave For Home" absolutely destroyed me. I cried and cried over that ending and never forgot it. The only TZ episode I refuse to watch again. The misery in that man's voice when he uttered, "I want to go homb68e," will always stay with me.
If you love Jesus Christ and are 100% proud of it, copy this and make it your signature!

Spur Of The Moment.
Trying to alter a crappy future.
On Thursday We Leave For Home.
"I want to go home."
And When The Sky Was Opened.
Must be the ultimate nightmare.
Dear Notcha-hooha, mookie mookie hama. Kau-kau manua.