Legal drug: Nicotine or alcohol
Illegal Drug: Marijuana or cocaine
These results were from the US and Canada, answers could change if a broader study was available.
Alcohol is the oldest drug known to man, and also the most widely-used drug. So, I would imagine it is the most widely-abused drug, as well.
Alcohol Marijuana is the most abused illicit drug in the United states army
Alcohol
Alcohol.
Marijuana.
alcohol
cocaine
Sugar
oxycontin
pain pills or marijuana
Marijuana (weed).
There is no definitive measure to determine the addictiveness of a drug. However, Xanax is considered the most abused psychoactive drug.