Gaga, and it's not even a competition.
Rihanna holds the digital record, but Lady Gaga & Katy are close to outselling her with only two albums worth of singles compared to Rihanna's 7's worth(more if you include the features)
Gaga sells in bulk - Katy does as well, but with the discounts. But in general, Gaga is the bigger star and sells more singles in general, so Gaga is the answer
