We recently saw one in my Psychology class, but I saw it coming yet still didn't turn away. I was 90% sure it was a screamer, but that 10% kept me looking, and sure enough, it was. But that wasn't the scariest.
The scariest one I had was the first time I ever experienced one. My sister sent it to me, and it said, "Look closely at this room and you'll see a ghost" or something. Would have been pretty obvious to anybody who had seen one before, but as I said, this was my first time and I'd never heard of them before. I had the sound pretty far up and it was like one full minute until the scream, and I had my face really close to the screen, and then it happened, and I literally fell of my chair and started screaming "SHIT SHIT SHIT SHIT SHIT". By the way this was back in like 3rd grade. Crazy stuff.