In retrospect, it was probably back when i stumbled while walking down a mountain and fell into a thorn bush. At the time being i was occupied with the pain of having a million thorns stuck in my arms, but when i think back at it, i could just as easily have tumbled a kilometer down the mountain.
._.
It still gives me the shivers. I could have fucking DIED there. My other "scariest" moments are just power outages and nightmares. The worst nightmare i've ever had had me trembling in fear for hours after waking up. That was back when i was a wee lad (5 years old or so) though, and the content of the nightmare doesn't appear frightening to me any more.
And for a more lighthearted moment, the first screamer i witnessed, with a former friend. (he's a dick now)
Basically, he told me to put my nose right up against the monitor. And i was standing up. The scream came, and i jumped at least half a meter in the air and fell flat on my ass/back. The shock was so great that it didn't even hurt. Then we both lol'd. :U