Scariest ghosts in fiction

The heart in Tell Tale Heart

Miss Jessel and Peter Quint from The Turn of the Screw

The Overlook in The Shining

The cat/kid hybrid from The Grudge & Ju-on

The mother from The Grudge & Ju-on
(I can make the sound she does and scare the sh*t out of my hubby - super fun.)

Sadako from The Ring

The little girl from The Sixth Sense

The twin girls from The Shining

The slit mouthed woman from Carved or A Slit Mouthed Woman

The sack wearing child from The Orphanage

Best fictional ghosts

The ghost in Hamlet, Hamlet's father.

The three ghosts of Christmas: Christmas Past, Christmas Present and Christmas Future.

Casper the Friendly Ghost.

Bloody Mary.

The Amityville Ghosts.

Beetlejuice.

Marshmallow Man.

Dr Malcolm Crowe.

Jacob Marley.

Headless Horseman.

Inky, Pinky, Blinky and Clyde.

Slimer.

Sadako.

The cat/kid hybrid from The Grudge.

Out of protest because I can't stand overly schmaltzy romance, I've left off the Patrick Swayze ghost. Also, I don't consider it a very good ghost character, it being almost exactly the same as a live person.

A good ghost should be shocking, unnatural, gross or appealing. They often appear in loops (residual echoes), illogically justify their existence, have a message or a mission for or against the living.