That would depend on how you define rare drug.
Maybe 2-methyl-2-butanol, though it isn't actually such a rare chemical. It just hasn't been used as a drug much since the early 1900's, and seems to have been pretty much forgotten as a sedative, only being used as an industrial chemical in modern times.
When it comes to the difficulty of obtaining the chemical, probably 2c-p and 4-ho-met. Neither has been very available (if at all) in quite a while, to the best of my knowledge. LSD is also pretty hard to get, though not nearly as much so as 2c-p and 4-ho-met. I could pretty easily get LSD. It is just too expensive compared to other psychedelics I find to be just as good.