Perhaps not scary but twisted would be funny games. 😵
Put her in a dark room with headphones and the game Amnesia, that should do it.
For me the truly scary movies would be ones where no special make-up or special effect is involved. Something that can "possibly happen" in real life. There's 2 sliding door mirrors for the closet in my room. If I watched paranormal activity, I would not be able to sleep at night.
The Ring is good especially if your phone rings right after the movie is over. 😉
I don't know that I have one scariest movie, but some recent favorites include The Descent (this also happens to be a movie women like because the whole cast is female), Insidious, [REC], and 28 Days Later/28 Weeks Later. I found the Paranormal Activity movies very scary as well but some people find them too deliberately paced.
Rosemary's Baby is my favorite. Make sure you serve chocolate mousse for dessert.