Forbes is the only list that maters. Single releases, album releases, music video releases, tours... The only reason behind all this is the effort put by one's team in making money. This way, the most successful artist is always the one that makes more money.
Despite being a radio and digital beast, Katy Perry fails to be a trademark as profitable as Lady Gaga - maybe because she isn't a touring force like Gaga -, and that's why she recognizes Gaga is the queen.