I think about this a lot because I attempt to write scary YA.
The Call, definitely. I think that’s the scariest modern YA I’ve read. The whole concept is just so freaking disturbing. I love Patrick Ness’s books but don’t find them scary per se. Anna Dressed in Blood has some very creepy moments, though for me the scares were mostly in the first half.
Amy Lukavics and Kim Liggett seem to have a lot of fans; I haven’t read them yet, but the books look scary, with supernatural elements. My debut is about a serial killer, based on a real one, with a slight spec angle. But I tried to make it gritty and psychologically disturbing while leaving the gore off-screen, which doesn’t work for, well, a lot of horror readers. My new WIP has more gore and a higher on-screen body count, but whether that actually makes it scarier I have no idea. (Personally, I don’t mind gore, but the stories that really scare me suggest more than they show, like The Haunting of Hill House.)