Some of the scariest films I've ever seen are horror movies that feature demonic possession. Those that immediately come to mind for me include
The Exorcist(1973),
Amityville 2-The Possession(1982) and
Beyond The Door(1974). Early next year on January 6th Paramount is releasing
The Devil Inside, directed by William Brent Bell. Judging by the trailer it looks promising. Check out the trailer below: