Im gonna have to say it's a tie between "Somebody That I Used To Know" or "We Are Young" in my opinion. Last year it was hands down "Runaway" by Kanye West.
if it's 2011 included, I'm gonna go with this instead--- <iframe width="560" height="315" src="http://www.youtube.com/embed/-yg3dLEDWqs" frameborder="0" allowfullscreen></iframe>