my creek boating days are basically over but for me scariest rapids I have run in no particular order:
Whiteline on the Poudre. I don't know what the flow was, it was on the way to run Big South and for some reason this rapid always intimidated me. Maybe just because it was roadside and I looked at it for too long.
Supermax at 1000cfs. Maybe it is not as hard as it looks but "scary" is a subjective characteristic.
Cool World and Double Trouble on Big South. I have never had a bad line in either, but they were always scary for me.
Lake Creek. Lake Creek falls in a differnent category of scary for me. Cold. High alpine. I learned to paddle on the Ark and so I can remeber going up there my first season paddling in '93 and thinking people had to be insane to run that. I have only ever run the sneak on river right at Paralyzer so it hardly counted.
I have never run many of the classic hard runs in the State so my list is probably woefully incomplete. Cutch or someone from that crew can do a way better job than me ranking rapids....
One of my runs through Double Trouble. I think Menges took the photo.