The Walking Dead - Negan and Lucille
"The Day Will Come When You Won't Be" is horrifying because death is looming for a few characters and having to go through that with them stays with the audience for sure.
The Vampire Diaries - Katherine Appears
The Vampire Diaries in its early seasons was much scarier, and it worked exceedingly well. Moments like Katherine showing up and killing Uncle John when he thought she was Elena? That is all kinds of iconic.
Twin Peaks - Killer BOB Introduction
Twin Peaks is confusing on its best days which is why it is essential to watch it with an open mind. But what really sticks out has to be that Bob shot where he comes at the camera, terrorizing the viewer from there on out.
The Exorcist - Casey Falling Victim to the Devil
The show was an anthology, so each season had a different possession, but that first story has to be ranked the scariest. The Exorcist was still starting so you had no clue what approach they would be taking, so every hint of someone being possessed and displaying those chilling signs worked very well.
The X-Files - The Peacock Family
It feels ridiculous even to mention which episode is the scariest, but it only makes sense to pay "Home" some well-deserved praise. Based off a frightening story of a real family similar to the one portrayed in this episode, Mulder and Scully discover a terrifying secret in that house. Any scenes after the truth are shared with them and with the audience has the potential to be terrifying
Black Mirror - VR Plot Twist
Playtest explores a thrill-seeking guy who decides to test a video game that is very advanced. The concept of VR begs the question of what is real and what is not, only for the ending to punch you when you least expect it. Black Mirror delivers above and beyond like always.