indeed, "most addictive substance" is a very difficult, and largely empty, statement to make. For instance, research recently showed that addiction to marijuana is worse as far as withdrawl symptoms (subjectively described) than cigarettes, yet nobody would, in their right mind, claim that marijuana is as addictive as nicotine.
Like anything with drugs, the statistics can be put in different ways in order to make whatever point you want, and imho, there is a significant lack of empirical research into drugs that isn't focused on saying "this will do X to you, oooooooh scary" or "this is how to treat addiction to X". Seratonin and LSD, something that is on the cusp of neuro-chemical research, is an example of the way all drugs need to be studied, as, again imho, they offer something akin to repetable and controlled lesions, which are one of the cornerstones of brain research. As it stands now, when the Lancet, one of the world's most trusted medical journals, published independent rankings of the harms of drugs (to compare them to the classification system of the British gvt), the opinions of clinical psychologists about the harms of drugs were more important than strict neuroscientific evidence in their rankings.
lol, to rant some more, and hopefully tie this all up, saying "heroin is more addictive than meth" is largely pointless, most significantly because there is not agreement in what consitiutes "addiction" (DSM classifaction vs behavioural measures like withdrawl effects), but even further, because people use different drugs for different reasons. That most ravers have done meth in pills or at a party for a buzz is much different than those in poverty who use heroin as a form of escapism.
imho, the environment of people is far more of an influence for addiction than the chemical substance in itself.