Gaga. Keep clinging to album sales from the 90s/early 00s when they were three times as big as during Gaga's time and when pretty much anyone sold 15 million+ effortlessly
Bigger album sales: Britney (because of the market was insane during those days, any pop girl from those days have outsold the current girls)
Bigger singles: Gaga by far
Bigger tours: Gaga by far (also because the touring market increased)