Rarest in the pet trade? I don't know. Probably one of those tropical Cherax. Definitely not the "electric blue lobster", which is usually a blue morph of the Florida Crayfish (Procambarus alleni) and is one of the most common crayfish in the trade. If you are looking for a species in high demand to breed, I suggest the orange morph of the Mexican Dwarf Crayfish (Cambarellus patzcuarensis). Lots of people seem to be looking for those guys.
There are several hundred crayfish species altogether, and the great majority are not in the pet trade at all. There are quite a few crayfish species that are critically endangered in the wild, especially spring and cave associated crays that often have tiny ranges.
There are several hundred crayfish species altogether, and the great majority are not in the pet trade at all. There are quite a few crayfish species that are critically endangered in the wild, especially spring and cave associated crays that often have tiny ranges.