Very hard question!
When Princess Diana was alive, she was, unquestionably the most famous and popular royal. She had an immense global reach.
But there is no such personality now. The British royals are the ones most people are familiar with. But I doubt there is anyone who generates much interest internationally and that includes Prince William. Although that might change once there's talk of his marriage.
As for other royal families, you'd be surprised how many people out there havent heard of most of them. Even well-informed people may know about the politics of various nations around the world, but not much about their monarchs. They just dont elicit much interest. And that is understandable.