Depends on how you look at it.
From an overall cost to society: probably alcohol.  Many, many people drink and I'm sure everyone has commited at least one titanic f*ckup when drunk.  Factor in lost work time, illness, treatment, impact on families/relationships, etc.
In terms a drug's ability to ruin your life in very short order: crank/crystal meth.  Users become addicted, on average, the third time they try that crap.  Once hooked, they need to do increasing amounts of the drug to get the same effect and eventually fry their brains, have a psychotic break and kill or injure themselves and/or others in a fit of paranoia.  Even if you do manage to kick the addiction, you'll likely have brain damage for the rest of your life.  That stuff is tha debbil.