Gaga was the first musician to reach one billion views on YouTube back in October 2010, but according to Billboard "has failed to sustain the hundred million-view pace of tracks such as Bad Romance and Telephone with more recent efforts such as You And I and Marry The Night, which still top 49 and 31 million, respectively."
Rihanna's We Found Love has received so far more than 115 million views after three months. However, Justin Bieber is still the YouTube frontrunner, his videos earning him more than 2.2 billion views.