Like he wasn't huge at Man U?
I'd say Ronaldo, Messi, Obama or the Pope. Over three BILLION people watched the 2014 World Cup, and Ronaldo is the most recognizable face in soccer right now. I would guess its him. If 3.2 billion people watched the WC, that means 3.2 billion people decided to watch soccer and the biggest star would no doubt be noticed by those people. One billion people watched the WC final alone, so they were exposed to Lio Messi.
So I'll say Ronaldo, with Messi and Obama being slightly behind. If you showed people pictures of those people NOT in official garb like the Popes hat or the Queens jewels, I guarantee more people recognize CR7 or Messi's face.