I was actually being serious with my Mickey Mouse suggestion. I believe that he is the most popular figure in the world, though the prompt was technically for a living person. On the whole, symbols are well-known, but people aren't necessarily able to be universally known like Mickey Mouse is because they have specific cultural appeals.
Besides Mickey, Barack Obama is, I think, the best answer I've heard so far. This is an American figure that is wildly well-known in both modernized Western countries and 3rd world African countries alike. (I'm not sure about the other countries, but I think it's a safe bet that he's well-known there too.)