Likely the most potent species are the wood chips inhabiting species like
Psilocybe azurescens, Psilocybe cyanescens and
Psilocybe subaeruginosa.
Similar in the alkaloid content are the dung inhabiting species
Panaeolus cyanescens and
Panaeolus tropicalis.
Also very potent is the pasture growing species
P. semilanceata but the cultivation of the later is extremely difficult.
Psilocybe cubensis is slightly less potent but the easiest to cultivate, that being the reason why it's so popular.
The exact alkaloid content varies greatly even from one specimen to another withing the same species.
Related document:
Magic Mushroom Dosage Calculator
