I am another one who benefited from a liberal reading philosophy by my parents, who let us read way age-inappropriate books and novels. I was pretty young when I discovered Stephen King, probably about fourth grade, but a lot of his novels scared the crap out of me. I remember both "Salem's Lot" and "The Stand" as giving me nightmares.

"One Second After" did nothing for me, as it felt incredibly derivative of "Lucifer's Hammer", by Larry Niven, from the 80s. Right down to the semi-cannibalistic army driven by street gangs. "The Passage" also felt a little bit like a pastiche of better written books the further I got into it.

I don't read a lot of zombie-fiction, but I picked up a copy of "The Girl With All The Gifts" at an airport book shop a few months back, and felt it was well written, and the ending gave me some pause. These days, that is about as close to a scare as fiction gives me.