Your Premise is All Wrong!

Your question is based on a false premise, namely, "what are the worst and what are the least dangerous drugs..." For example, I have known heroin users, who used for 30 years without an overdose - including a favorite cousin of mine. Am I saying that heroin is not dangerous? No! 

More recently, I met the 20 year old brother of a middle school girl, who attended a family counseling session. He was in a helping profession himself, and never used recreational drugs, until one night when his new girlfriend gave him a single tablet, probably what is now called 'Molly.' Molly is supposed to be the 'relatively less dangerous' drug 'Ecstasy' or MDMA. Unfortunately, 'Molly' often contains all kinds of dangerous analogue drugs, and this strong 20 year old man felt ill, called his mother to pick him up from a large warehouse party, and went to the hospital. He suffered brain-death that very night, and was taken off the ventilator by morning. The toxicology report was inconclusive. True story.

The issue is not which drug is more dangerous, the issue is that you do not know what you are actually getting on the street unless, quite frankly, you know the chemist. I have known heroin users who were intentionally killed with a 'hot shot,' poison of one sort or another sold as heroin. Even LSD-25, which is not physically poisonous or addictive, despite the bad rap its gotten over the decades, is being replaced by a new substance, 25I-NBOMe, which also works in extremely tiny amounts, is sold like 'acid' as small blotters, but IS toxic, and HAS resulted in a number of deaths, according to Wikipedia.

The slogan, 'Just Say No,' might work in elementary school, but by middle school, it is useless (I have 27 years of experience as a substance abuse specialist in middle schools). A better slogan is 'Just Say Know,' and by this I mean that young people need to know not only what the effects and hazards are for recreational drugs, they have to know that in the past few years the risk has gone way up because of the the substitution of 'relatively less dangerous' drugs for some very dangerous drugs.

There is nothing I could recommend in this case. Marijuana has not killed anyone directly by overdose to my knowledge, but people have driven while very stoned and killed others. If one knew that Psilocybin-containing mushrooms were actually that, and not a poisonous look-alike (inebriating 'Liberty Caps' sometime grow right next to deadly Galerinas, and look identical to the untrained eye), then toxicity wouldn't be the problem, but a hallucinogen-induced psychosis, or dangerously reckless behavior might still remain a danger. Pharmaceutical drugs are the cause of more overdose deaths than anything else at this time, and should be avoided for multiple reasons, but purportedly 'less dangerous' street drugs might turn out to be a whole lot more toxic than one expected, with tragic consequences.