What's wrong with Ketamine? It's not particularly addictive, and it's worst side effect is kidney damage (which can be quite severe). Are you sure you don't mean Heroin or Fentanyl? Those are the worst of the bunch.

By "most harmful" I meant total harm to society, not personal harm per microgram or something.

How many families have been torn apart by alcoholism? How many drunk driving deaths? And yet it still doesn't get the stigma of being "drugs" - this is the bullshit I'm referring to.