I'm not quite sure if something like "the best" medium exists, because microalgae can be cultivated on several media (especially Chlorella species, which are typically rather robust). The choice of medium will also depend on what you really want to achieve: one medium may be better for rapid algae biomass production, while another may be better to improve the nutritional value of the biomass (which will depend on the nutritional needs of daphnids).
In any case, you could check the "SAG basal medium" for a general medium that can be used to grow Chlorella species: http://sagdb.uni-goettingen.de/culture_media/26%20Bold%20Modified%20Basal%20Medium.pdf
Hopefully this already gives you a good lead! That said, I would definitely also check what other labs with a record in Daphnia research use - the composition of the algae medium should ideally be mentioned in their papers somewhere.