i think kakashi once said the byakugan was better didnt he? im not sure so dont fire hatred at me i just have a vague recollection aha but would depend on the circumstance e.g. whoes sharingan is it? because madara and shisui had space time jutsu with there sharingan so they could just teleport behind a person with the byakugan and stab them in there blind/weak spot. but if its in the dark then a byakugan wielder due to them being able so see everything and the sharingan would be useless due to they couldnt see anything and also the byakugan wielder couldnt even look in there eye.
also genjutsu is pointless against a byakugan wielder because they can tell when there chakra flow is altered and then they can brake it so in short:
byakugan beats sharingan unless it is space time jutsu sharingan like madara or shisui