I don't know how anyone could pretend to have an authoritative answer even if you give some objective definition to 'most popular'. It's not like there are easily comparable usage statistics being collected with any degree of accuracy. "It's all about who you know."
But to hazard a guess: probably some really tiny town that happens to have a small group of friends who trip regularly. LSD is rare and obscure enough that the proportion of the population that uses it regularly is almost certainly in the single digit percent range, if not a fraction of a percent, so a community of 60 with 3 trippers would probably win in terms of ratio. If you change the standard to raw consumption level instead of per capita, probably LA or NYC.