Answer:
The batches of the generic for Xanax, called alprazolam, made by a number of pharmaceutical companies, can vary in potency from 85% as potent as brand-name Xanax, to 110% as potent. This is the FDA allowable maximum varation, in most cases.
There is no "best brand of generic Xanax" per se, even though some people will say otherwise (due to their own perceptions, usually, rather than any scientific study).