As a person and a celebrity , definitely gaga , but if we are talking about whose music is bigger , then rihanna by miles , and since the latter criteria is more important to a singer , I picked rihanna
From debut album The Fame, Gaga was in the top 10 album charts in 26 countries.
Add to the fact her twitter acount recently hit the 16 million followers (and counting),
She has caused an impact on media as in for her style, excentricity and her talent.
Even though Born this way sales were lesser than The fame, this album has reached up to 33 countries being at least 1 week in number one sales.
On sales, Rihanna defintely surpasses Gaga, but still, Gaga has the fans, The Fame background, the support of the media,and is not going nowhere soon.
Australian Albums Chart 5
Belgian Albums Chart (Flanders) 20
Irish Albums Chart 2
Polish Albums Chart 22
Swedish Albums Chart 31
UK Albums Chart 1 US Billboard 200 4
Joanne, simply because of Asia. Asia is a fickle market. Avril, Lady Gaga and Britney are the only current artists who have ever had substantial success there.