10The Forbidden Forest
At the edge of the grounds was the Forbidden Forest. It was said to have harmless creatures such as unicorns, a little more dangerous beings such as centaurs, and basically everything all the way up to werewolves. In 1991, it was even home to Lord Voldemort for a while.
Places don't get much more dangerous than that.
And, to the school's credit, it was mostly forbidden. Except when Harry, Malfoy, and Hermione were given a detention in the forest and had to go in there when something clearly dangerous was lurking. Which turned out to be Lord Voldemort. Er... good call, professors.