It depends on the situation. Some are highly addictive and can destroy your life while others are not addictive but do massive bodily harm. Methamphetamine is an example of both along with heroin and crack but that is circumstantial.
all drugs are evil for addicts. In my opinion PCP is one of the most dangerous because it brings the mind to a whole new realm. the user has no conscious thought process. alcohol is one of the worst only because it is socially accepted.