This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / No Country for Old Men
Meet Anton Chigurh.
It's hard to find individual moments, but in general: Anton Chigurh is the scary foreign Terminator. Everything about him is unsettling. On top of that, there's the bleak, eerie film noirish atmosphere, even in the daytime. And then there's gunshots and blood everywhere, and special mention goes to Anton's captive bolt pistol.
The fact that he's still at large when the film ends doesn't help at all.
It's pretty safe to say that any scene featuring Chigurh is bound to leave viewers clenching their teeth. Wherever he goes, death seems to be not that far behind, and that is only made worse by the characters who don't know who they are talking to.
The infamous coin toss scene, where Anton decides whether or not to kill a local gas station clerk. Just the thought of having your fate decided by Heads or Tails? is unnerving.
Keep in mind to in regards to the scene: All the clerk did was start small talk, and Anton began to dissect everything about his life and his choices and deciding if he should live or die on a coin toss. The clerk's confused and downright terrified reaction is unsettling. Even when the clerk's choice lands him to live another day, Anton still has to frighten him about the coin is 'special'. Makes you wonder if Anton was gonna kill him anyway.
The Car Accident scene in which you would see Anton's broken bone sticking out on one of his arms.
Anton's confrontation with Carla Jean. She calls out his Blue and Orange Morality logic and has Anton decide her fate without the coin. Cue Anton's completeVillainous Breakdown, with subtle movements as he bluntly tells Carla Jean to "call it".