This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Cloverfield
Face to face with nowhere left to run.
Giant tick-things appearing to grin, with eyes glowing in the dark.
And the octane rating only goes up once you get bitten.
"I don't feel so good."
Much like how the original Godzilla was a metaphor for the horrors of Hiroshima and Nagasaki, there's a good amount of terrifying 9/11 imagery here.
The fact the movie leads with a title card reading "...retrieved from the area formerly known as Central Park."
They're not revealed in the film itself, but you see those tube-like things dangling from Clover's belly? According to behind the scenes material, those are essentially elongated mouths with teethlike fingers that lead into Clover's stomach. Anyone unfortunate to be running underneath them are sucked up and eaten alive. Imagine running for your life, only be sucked up by those things. Or even worse, imagine running with your family, only to watch them get sucked up.
The exact fate of Marlena. While the main characters are walking through a Sinister Subway tunnel, a scene which is pretty creepy in its own right, Marlena gets bitten by one of the parasites that fall off of the giant monster-thing. A few minutes later, when they end up at a first-aid center set up by the Military, her eyes start bleeding uncontrollably. Everyone starts freaking out, they drag her behind a curtain and we see a huge splatter of blood against it. We explicitly see her body rapidly bloat up like a balloon and explode.
"..which means they're going to let this whole area go..."
"You mean Mid-Town?'
"No, I mean Manhattan."
Just consider how bad things have to be for the government to even be willing to flatten all of Manhattan. The entire situation can be summed up by one line of dialogue from the soldier that rescues Rob and his friends after they ask him what Clover is.
Soldier: If they know, they ain't telling me. But whatever it is, it's winning.
The scene where Hud comes face to face with the Monster. We don't even get a cheap jump scare, he just stares into the camera, takes a sniff, then CHOMP.
Hell, the whole movie is goddamn terrifying. It is extremely relentless, realistic, and worse yet, it exactly feels like you're caught in the middle of a monster attack.
Clover itself is scary, but the scariest part of it all? It's a baby.
Babies spook easily, and when Clover gets skittish, buildings crumble. Worse...
There's also the fact that babies have enormous appetites, and in between spooking and running away, Clover has to stop and feed. And he's surrounded by tasty little treats - us.
Clover's roars are the stuff of nightmares but special mention goes to where the Hammerdown Protocol rains down on him offscreen. We only have to think about the type of pain it's going through to cause it to unleash a never ending scream of agony.