I'm only able to come up with the leaders of 12 countries: Canada, US, UK, Russia, Germany, France, Israel, India, Cuba, Syria, North Korea, and oddly enough New Zealand. I am from Canada.
There were many other leaders I was familiar with but they are no longer in office. There were also leaders who I would know if you told me their names but who I was unable to come up with myself.
If I were to guess the most well known leaders in order I would say Obama is first, Putin is second, but beyond that I am not sure. Maybe Merkel or Cameron would be third.