Oprah may be broadcast in Mexico, but she's practically unknown there. I was living with a well-to-do family in southern Mexico, and my host mom was constantly glued to the tv (this was in 2001). She was watching Beloved on day when I mentioned, "Oh, that's Oprah." She had no idea who I was talking about.
I think a much more accurate way would be if people were shown pictures, would they be able to recognize who it was.
QEII obviously takes that, hands down. She's known as the "The Queen" colloquially and not necessarily "The Queen of England" for a reason. She's on the currency of almost all of the commonwealth nations, and is extremely well known in Europe and the US.
Madonna is very famous world-wide and has been for a decades. I used to have a pen-pal in China (late 90s) who spoke very broken English, but was absolutely obsessed with Madonna.
And then there's Hillary Clinton, who was famous to begin with, but whose profile has been infinitely raised by being Secretary of State. Her constant visits abroad put her on TV in every nation she travels to.
I would deny Angel Merkel because she is not an iconic figure and most people probably couldn't pick her out of a line-up. In the past, I would have suggested Julia Roberts because she looks distinctive, but her appeal has faded over the past decade.
I would also put Jennifer Aniston as a contender (in a top 10), given that Friends is/has been extremely popular virtually everywhere it has aired (and yes, that includes China) and she has continued with movies. And again, she is distinctive enough looking to be recognizable.
Barbra? Please. She is talented yes, but I'm sure that she's never been a radio staple in Peru or China. As for JK Rowling, I don't even know what she looks like.
As for Bollywood, it's extremely popular, but really only in India. While that is a big market, it's still just 1/6 of the world. Ditto for Chinese TV/movie stars.
If we're talking living or dead, then the Virgin Mary, followed by QEII.