Poltergeists are scarier than normal ghosts like Casper (well, most anything is) cause they can manifest themselves in the most creatively horrific ways.
Who didn't say "HOLY SHIT?" when they first saw this movie? Directed by TEXAS CHAINSAW MASSACARE's Tobe Hooper, POLTERGEIST could have easily been another AMITYVILLE HOHUM, but the numerous ways this evil entity inflicted its wrath on this hapless family was amazing.
Sucking their daughter into a television, making the front yard tree come alive, animating a stuffed clown toy, fooling someone into thinking their face was falling apart, and making giant white spiders with skeleton heads appear was only the tip of the iceberg.
In the sequel, POLTGERGEIST II, ALIENS artist HR Gieger, designed some of the creatures, including a tequila worm that gets thrown up and grows into one of the most disgustingly disturbing things you'll ever see on film (aside from Craig T. Nelson).