The music video for the song "Gangnam Style" by Psy was the first video to exceed one billion views on Youtube. This landmark was reached in December 2012, and the video was getting about seven million views per day at the time.
In June of 2014, about two years after the video was originally posted, it hit two billion views.
The only other video to have passed one billion views as of April 2015 is Justin Bieber's "Baby," and the next most-viewed video is Katy Perry's "Dark Horse" with just over 900 million views. A number of other music videos have exceeded 800 million views, including Psy's "Gentleman."