I vote Nemisis Inferno, Thorpe Park, England. Once got stuck on it cause some fat lady's restraint came off when it left the station. She didn't die or got hurt or anything, but it was still annoying waiting there for 30 minutes...
*EDIT*
Unfortuantly, I have to change may vote cause I am not allowed to nominate a coaster that has already been nominated (maybe this could be more clearer in the rules?) Anyway, I vote for Saw the Ride, Thorpe Park, England. Been on it a few times, and it is always fun to act casualy around the life-actors while a group of teenage girls behind you scream their head' s off!