I'll say each has been having some type of success that could easily please their fans, however my vote would go toward Rihanna. Though Bey's album isn't out yet, I would still say Rihanna's is the most consistent selling while her singles are peaking at great positions.
Regardless of what Soundscan says about FF, I'm very pleased with Britney this era. All of her singles have become hits, with TTWE being a massive one, and her tour is doing well in addition to having rave reviews. All I can do is

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