Scariest game ever? Easy.
The scariest game I've ever played is Temple of Apshai on Commodore 64. The original from Epyx, not the remade Trilogy from Westwood.
You'd be wandering through a dungeon so dark, you can only see blackness and grey walls, and only the objects in the part of the room you're standing in.
The only sounds are your own footsteps and, much quieter, an eerie filter-swept bass note that would slowly ascend and descend in odd scales. Then, suddenly and without warning:
The most horrible cacophony you've ever heard in your short six years scares the living bejezus out of you! A terrifying monster is about to attack you! The music starts pounding out loud and strong: