What is the most addictive drug?

Although dependency on hallucinogens like LSD otherwise known as "acid" and psilocybin is listed as Substance-Related Disorder in the DSM-IV, most psychologists do not classify them as addictive drugs. Experts on addiction say that the use of LSD and psilocybin causes neither psychological nor physical dependency . Many users report feeling less desire to use these drugs after every use .