Scariest is probably the High Roller in Las Vegas. It's not fast or long but it's attached to the side of the Stratosphere Tower and is 1000 ft up in the air. I hate heights (yes, really) and hated every minute of the ride - felt like I was going to plummet to my death. My favourite coaster at the moment is probably Balder in Liseberg, Sweden. It's made out of wood and is simply stunning. Lots of airtime and rattles :)