I second Dracon's bet on "Behind Jaggi Lines". Nothing made me scare more on any computer and in any game... Even Quake and Hexen on PC were nothing compared to the Jaggi monster... Here is the guy (from game review on this site):
Definitely the Jaggi monster. I also remember the sound accompanying the monster showing up. I was scared, but you would have to see my brother (11 years younger). He was in a real panic !!!! :-)
Nothing ever scared me on Atari, probably due to fact that I missed Behind Jaggi Lines game. One of the first games I played on my first PC computer was Quake in 1996, and this (watch the movie) monster scared me almost to the dead: