This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Carnosaur
Even with the ridiculous premise and some of the stiffest, most unconvincing dinosaur effects ever not to be produced by the SyFy channel, the first film is still a very moody, very ominous, very bloody piece of work.
The scene where the Deinonychus finds a group of hippies who chained themselves to bulldozers and viciously mauls all of them. What's worse, all of the group could hear their friends being torn apart but are both completely unable to help them or free themselves to get to safety. The thought of being trapped in an isolated area with a monster ripping your party apart one by one while you can't get away is genuinely terrifying.