This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / The Thing (2011)
The Head-Merge scene. It is incredibly disturbing, and the fear of the victim is very horrifying to see.
The infection of Jonas right after is just as bad, if not worse. He's helping drag the unconscious Edvard into the rec room, when all of a sudden, Edvard's arm detaches from his body, and starts crawling up it's victim's arm. The fact that you can hear his muffled screams as the Thing latches onto his face does not help. Even worse, just before Kate burns him, you can start to see one of the thing's tentacles push through his cheek and one of his eyes. And when the Edvard-Adam Thing appears again, the two faces unnaturally, like they're trying to pull apart, and seem to be frozen in a scream.
The Helicopter scene. One would think it's the one who was splattered with blood would be the Thing. However, we look up, and then we see his partners face crack in half, realistically.
The scene where Kate realises that the woman standing right behind her (between Kate and the door) is the Thing.
The sounds the humanoid Things make are terrifying. They are almost totally alien yet still have just enough of a human quality to them to remind you that this was once a human being (or several).
Consider this. The Thing can take the form of any life form it comes into contact with. It's very likely that if it was released into a populated area it would bring about the end of the world as we know it. You know that thing we see burst from the ice block it was contained in? It hasn't been confirmed whether that in fact it was its true form. One could make the theory that the form we saw was that of the Thing's last victim prior to crash landing on Earth.