I read the actual report a little while ago and some of you are missing the point. The list is not just what is the most harmful to the user, it also factors in what is the most detrimental to society in general.
It takes into account, harm to the user, harm to the people they come into contact with (friends, family, workplace, public), harm to property, loss of production etc.
It is the harm to society that the substances pose that the ranking is based upon, not just the harm to the user themselves.