Only one drug? Really?
Unfortunately, the criterion is too specific for me. The tedium and sateity experienced by a dearth of psychoactive variety would be unimaginably insufferable. Morever, my tolerance to most drugs spirals uncontrollably in an upward trajectory—a sort of quasi-tachyphylaxis, one may say—rendering the drug both exceedingly boring and frustratingly ineffective.
How about just one class of drugs? If this exception is permissible, then the query becomes more answerable.
I'd choose, withiut a moment's hesitation, GABAA Receptor positive allosteric modulators (GABAAR PAMs) as my final and permanent choice.
If classes, families, or types of drugs are not allowed, then I guess secobarbital or either triazolam would be my decision.