I'm sure everyone here has good ideas of the scariest ghosts in video games, but here are some picks from me.
Kackle from Donkey Kong Country 2:
Chases Diddy and Dixie through a creepy old house and laughs evilly if he catches them.
King Boo from the Luigi's Mansion trilogy:
He wants to trap Luigi and his friends in paintings for eternity.
The Chozo Ghosts from Metroid Prime:
They relentlessly ambush Samus and aren't easy to defeat.
The Pokemon Tower ghosts from Pokemon Red & Blue:
Always found them very unpleasant when I didn't have the Silph Scope.
So what are the scariest video game ghosts to you?