Definitely Space Mountain It would be the scariest there, but I don't think of Disney as having really scary rides, definitely exhilarating and fabulous but not that scary.
Oh yeah, there was the time I went with my best bud (whose husband works there) and we got stuck on Splash Mountain for literally 15 minutes. It wouldn't have been so bad but we were stuck in the up position so our necks were killing us. Then when the guy came out of the side of the top of the mountain (where we were stuck) and told us the ride was shut down because THE CAMERA WASN'T WORKING I am very our spoken (understatement of a lifetime) and replied, ARE YOU KIDDING ME? (I didn't cuss there were kids)
The ironic part of the whole thing was even though she had been there hundreds of times (I'm not exaggerating) it was her first time on the ride and it took me an hour to get her to do it. We laughed when we got down, waited a little while and rode it again :))