This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / No Country for Old Men
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 Slasher Smile Anton makes when strangling the guard in the beginning of the movie, it is one of the very few scenes where we would see his expression.
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.
The Car Accident scene in which you would see Anton's broken bone sticking out on one of his arms