Anything with a higher risk-to-benefit ratio serves no purpose except destroying individuals and burdening society.
In my opinion, the following drugs are just unnecessarily powerful and/or ridiculously harmful:
IV heroin
Crystal meth
Crack cocaine
Tobacco (shouldn't be extinct, but should be more tightly controlled IMO)
Bromo-dragonfly
Inhalants, i.e. paint, duster, glue, gasoline, etc.
Fentanyl (so much more powerful than heroin, yet doesn't get half the bad publicity)
Alcohol (again, it has its uses but should be more closely regulated)
In my mind, if some of the "softer" drugs were legal and therefore purer and more affordable, use of these harder drugs would likely dwindle dramatically. Who is going to huff paint when REAL hallucinogens are easily available? Same goes for research chemicals, most of which have no history of human use and might well kill a person but are the only legal alternative for altering consciousness.
Now for the "drugs that should be decriminalized/regulated immediately" list:
Marijuana
Mushrooms, mescaline and LSD
Opium poppies
Coca
Khat
MDMA
Ephedra
^There you have a fine variety of stimulants, hallucinogens, empathogens and opiates to choose from, most of which grow out of the ground and have been shown to be benign when used responsibly.