What is the scariest American movie you've ever seen?

The Thing (1982)
Inseminoid. (1982)
The. Hitcher .(1986)
Pumpkinhead (1988)
Breakdown (1997)
The Astronaut's Wife (1999)
Funny Games U.S. (2007)
Eden Lake (2008)
The Midnight Meat Train (2008)
Dying Breed (2009)
The Stepfather - 1987
The stepfather 2 - 1989
The last house on the left - 1972 & 2009
The Human Centipede (First Sequence) - 2009
The Human Centipede 2 (Full Sequence) - 2011
The Human Centipede III (Final Sequence) (2015)
Hostel - 2005
Hostel: Part II (2007)
Hostel (2011)

The Evil Dead (1981) or The Thing (1982). Nothing made before or since tops these two.

The Exorcist or maybe Jaws.

Carrie. Based on the Stephen King novel. Scared the living bejesus out of me on several scenes in the movie. But the scariest scene of all was when her hand popped out of the ground into which her house had sunk. Eeeeeeeeeeeeek

Probably "The Shining" or "The Evil Dead". If you're looking for more recent movies, then probably "The Blair Witch Project" or "Insidious".

IMO the Blair Witch Project.
It turned me off of camping outside in tents.