"To Serve Man"
Some super-friendly aliens come to Earth and absolutely nothing goes wrong! Don't feel stressed out about watching this one at all! NOT. Let's just say the title of the episode ends up being a great "OH SNAP" moment in this one.
"I Shot an Arrow Into the Air"
In this episode, a bunch of spacemen land on a barren asteroid and shit gets nuts. Although if I were an astronaut on a foreign asteroid, I might be a little suspicious that I didn't need to have my space helmet on to breathe, but whatever. The '60s were wacky like that, man.
"The After Hours"
This one involves mannequins, being trapped in a store with mannequins, and possibly interacting with mannequins, so yeah, I'd say it's pretty horrifying.
"Five Characters in Search of an Exit"
If you're a Twilight Zone connoisseur, you can get kind of good at figuring out where the plot-line is going to go, but "Five Characters in Search of an Exit" is still a solid episode with a great "OH SHIT, WHAT IS LIFE" moment.
It's times like this that I wish the internet didn't exist (the only time I will ever say that); I feel like everyone knows the plot to "The Invaders" at this point, but if by some miracle you don't, watch it immediately and get ready for your brain to explode. Because it will.
"Nightmare at 20,000 Feet"
LOOK AT HOW HORRIFYING THAT FUCKING GREMLIN IS! I'm sorry, but this episode legitimately terrifies me. William Shatner (yes, that's him) is flying and a GREMLIN taunts him on the wing of an airplane.