Any series, any movie. I noticed that in TOS some of Rod Serlings Twilight Zone writers did write for Star Trek as well, so Trek always had some creepier, darker moments. I'll start with a few examples.
TNG "Night Terrors" - it really knocked me for a loop in the scene Dr. Crusher suddenly saw all these shrouded corpses sitting up.
TOS "The Empath" - many of those black-background scenes with the Vians were unsettling, especially when the crew got to the surface and saw that image of Scotty smiling at them, only to be an illusion.
TOS "Wolf in the Fold" - that seance was pretty dark and foreboding.
TNG - forgot the exact episode - that scene in which a female crewmember is killed by having her body reformed/bisecting halfway in and out of a deck. Was that a Q episode? I don't remember but I do remember that grotesque scene of a woman being killed by having her body made into part of the deck. That image stuck with me.
TOS - "What Are Little Girls Made Of?" - every scene with Ruk was obviously nerve-racking. Especially when he was lurking in the caves. Oh, and this includes Ted Kassidy's voice as Balok in "The Corbomite Maneuver" and the voice of the Gorn in "Arena." Let's face it, Mr. Kassidy had a naturally scary voice.
TOS - TMP - The long, dark, dimly-lit Entperise flyovers over the massive, gargantuan, utterly alien V'Ger craft, soaked in some of Goldsmith's darkest musical swirls.
TNG "Conspiracy" - the ending was as gross and horrifying as any David Cronenberg movie.
DS9 "Field of Fire" - many scenes with Ezri Dax, the image of a former murderous Dax host, and the mysterious assassin were very nerve-racking.
TNG "Night Terrors" - it really knocked me for a loop in the scene Dr. Crusher suddenly saw all these shrouded corpses sitting up.
TOS "The Empath" - many of those black-background scenes with the Vians were unsettling, especially when the crew got to the surface and saw that image of Scotty smiling at them, only to be an illusion.
TOS "Wolf in the Fold" - that seance was pretty dark and foreboding.
TNG - forgot the exact episode - that scene in which a female crewmember is killed by having her body reformed/bisecting halfway in and out of a deck. Was that a Q episode? I don't remember but I do remember that grotesque scene of a woman being killed by having her body made into part of the deck. That image stuck with me.
TOS - "What Are Little Girls Made Of?" - every scene with Ruk was obviously nerve-racking. Especially when he was lurking in the caves. Oh, and this includes Ted Kassidy's voice as Balok in "The Corbomite Maneuver" and the voice of the Gorn in "Arena." Let's face it, Mr. Kassidy had a naturally scary voice.
TOS - TMP - The long, dark, dimly-lit Entperise flyovers over the massive, gargantuan, utterly alien V'Ger craft, soaked in some of Goldsmith's darkest musical swirls.
TNG "Conspiracy" - the ending was as gross and horrifying as any David Cronenberg movie.
DS9 "Field of Fire" - many scenes with Ezri Dax, the image of a former murderous Dax host, and the mysterious assassin were very nerve-racking.