that would be this 3 i think:
The Cave in Kq4 it was dark and a big troll was chasing you..that would be i guess the scariest scene to play i all Kings Quest games.
And dont forget the chasm its terrifying to see not where you are walking and could fall down any minute that part was hard to play and scary.
One second place i would say the Ogres House.
If you entered the kitchen you not only saw a scaring scene, it was also dead end.
when you entered the house a dog wants to attack Rosella, you get an axe, and the bottom of that you must hide in some dark creepy corner until you could come out to get the hen, and then escape will you got the dog and the ogre in your back chasing you.
Third place i would say the old skull cave where you met those 3 old whitches with their glas eyes.
Remember what happened what the did do you when the caught you?
The last thing i can think of was the part in ooga booga, i dont know whis name anymore, but i meant that thing coming out of the earth with a twisted laugh and trying to catch Rosella, that was creepy too.
For me i must say those were there scariest parts to me in all Kings Quest Games.