hmm, that's an interesting question and one that might need a psychopharmacologist to answer it. do you count caffeine as a recreational drug? it depends how often the person would be using the drug, i think, i.e. every day over a long period or every weekend over a long period.
it's too individual to make a broad generalisation though, for practical purposes. someone could drink a beer, or a couple of beers fairly frequently and it could never be a problem. for someone else though, it could develop into a problem, whereas that first person may have had a problem with opiates if they started using them.
i don't think opiates are the safest because if you're talking about a long period of use then problems arise with tolerance, dependence and then withdrawal.
i would go with something that has the lowest addiction potential.