Beyonce is more well known. Everyone knows Beyonce.
But I'm not sure what exactly the question is asking. Are we comparing Rihanna and Beyonce on their success right now? Or over their whole careers?
If it is right now, Rihanna has the most success. She is pretty successful in the US, but the UK, Australia, New Zealand, and Europe take everything she does with open arms. So I would say that Rihanna is more international, which overall leads to a bigger amount of success right now.
If we are talking about over their whole career span, well... Beyonce. Anyone who says otherwise is deluded, tbh.