The Thing doesn't scare me so much as confound and entertain me to no end. Some of those effects are unrivaled.
I know it's been parodied to death, but one of the scariest initial viewing experiences I ever had was the Blair Witch Project. We might forget it now, but there was a reason that movie was such a huge success when it first hit theaters.
Alien (1979) How much more scary can a movie get? The underlying message -- the fright pysche -- Scary stuff is out there, stuff that we don't know about. We keep going out there and it is going to get us. Again: How much more scary can a movie get?
The alien monster -- one of our worse nightmares. In the movie, we first see the alien as this little monster thing breaking open John Hurt's chest from the inside, coming out of him, and scampering away like a squealing rat!
OMG! They can get inside of us and we wouldn't even know it! OMG! They're like rats! Getting into our house while we sleep!
Then the little ratty monster thing grows into a great big badass!
In life what we are afraid of --our fear grows, and grows, and grows, until we are like the crew in the movie -- little mice. And like the crew in the movie-- food for a monster and for her young. How much more scary can a movie get?