The first is drugs where dependence is chiefly psychological (alcohol, most psychedelics, marijuana, X). The second is drugs which have a strong physical component to dependence (nicotine, cocaine and derivatives, opiates). The last is drugs which induce violent and unpredictable behavior in the average user (meth, PCP, Ketamine) and are usually dependence forming.
The first category should not be illegal for any reason. However, I like the idea of denying welfare checks to folks who continually test positive for those drugs. I also like the idea of taxation at the state level to avoid such drugs becoming consumer staples. (Think of it as a luxury tax.)
The second category should be legal simply to reduce the criminal profit motive;criminal action from the second category almost invariably is a result of an addict not being able to find a 'fix'. Taxation proceeds from the first category should go towards providing rehab clinics with funding in order to get people off the junk once they go and get hooked (if they are the 10% that is susceptible to dependence).
The third category should only be available via the medical community and prescribed as necessary, but with general distribution regulated by the states. These drugs are so dangerous and unpredictable across the set of users that just allowing their use might present a clear danger to others.
I believe the above notes form a basis for a socially responsible and freedom enhancing drug policy.
Note that this is all contingent on getting the federal government out of regulating intrastate matters, and the idea that government should not regulate your own body, but is responsible for preventing you from doing harm to others through your poor choices. People are going to use drugs and fuck up their lives, just like they always have even within prohibition. Why waste money and make criminals of folks who only wish to harm themselves?