YSK one of the most thrilling rides is at Disney California Adventure; the ferris wheel on the swinging gondolas.

Now it's not that scary sitting down but if you get like 4 friends you can get a gondola to yourself so no-one snitches when you stand up.
The gondola swings about 60 degrees from resting position but it feels like its going all the way round if you get a good swing.

The reason why its the most thrilling is because you have no restraints. Then the slow build-up when the gondola starts sliding down, speeding up until you hit the swinging portion. Restraints give me a feeling of safety in the case of standard roller coasters. But in this case you're free to move around and are at the risk of falling over or something. Which is what makes it so exciting.

I know its against the rules but it's pretty safe inside the gondolas, we didn't jump or anything. Anyways just thought you should know what my opinion was for the scariest ride, hope it satisfies some thrill seekers out there.