This is a great answer, although I struggle to call a water slide a "ride". It IS terrifying though, I believe for all involved. I did it once... scared to death as I prepared for my decent. My wife who is a thrill junkie was too scared and waited by the exit. My ride down the slide was terrifying for onlookers too because when I got to the bottom and gathered my wits, I realized that my suit had rode up my backside pretty far terrifying all onlookers.
Evidence of my ordeal courtesy of my wife after having made slight adjustments...
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If I had to vote for a "terrifying" ride, I guess I'd have to say Mission:Space. This one also terrified my wife and she is not a weak person when it comes to thrill rides. I haven't been on the ride in a probably 17+ years. When I rode M:S it was when there was no "Green Team" and while in line the constant warnings for pregnant women, or people with health issues scared the heck out of my wife. By the time we got into the simulator and they were about to shut the door, my wife jumped up and said "I can't do this" and bolted. I was already set and was like ***! I couldn't react quick enough and before you know it I'm launched into space.
It's an intense ride for sure, and the first time you ride it, it's hard to know how much worse it will get as it continues. Anyway, that gets my vote. Btw, my wife got sh*t for years after that for leaving me to die on the spaceship.