I begged to see Poltergeist when it came out in 1982, I was 11. One of my main arguments was that it was only rated PG, ET came out the same year and it was rated PG; the PG-13 rating didn't exist yet.
I did not sleep in my room for 3 months after that and when I finally was able to go back all my stuffed animals had to be out of the room.
I have never found another movie again that scared me so much and I have seen more horror movies than I could count.
For me it was the multi-legged gargantuan monster at the top of the stairs, and something in the way the psychic described "the beast" that really struck terror in my poor preteen heart.