With most drugs being illegal, the current law criminalises users for purely having a good time. This creates a criminal underclass who are more willing to commit crime because they are already considered a criminal. In addition, as the drugs are illegal, criminal gangs are the only parties that can deal with the manufacture and distribution, creating a vast crime network. In addition, as most drugs are illegal, there are no safe content and consumption standards established, i.e. the fact that the drug is illegal makes it more dangerous.
In legalising certain drugs and establishing a legal network of distribution, many of these problems will be solved as well as freeing up the police and courts to deal with far mor harmful crimes. In addition, such a measure could tempt users of the hardest drugs to take up legalised drugs, after all, if you can get high legally, why would you take the risk of illegal and probably unsafe drugs.
In the frist wave of legalised and safety stamped drugs I would certainly proposed cannabis speed and ecstacy, followed by cocaine if it's safety can be sufficuiently established.
Of course, nothing is 100% safe but even excessive consumption of sugar free polos is bad for you (they cause the trotts) yet they are still legal.