By the time there is a need for a leader of the world the only people voting will be the leaders of each country. Individuals will have no say so in anything anymore except possibly to elect the leaders of their own country.
It would have to be Anders Fogh Rasmussenas, the current PM of Denmark, as Copenhagen and the country at large have the highest standard of happiness and quality of life. Personally, I like the way the Netherlands conducts business, focusing on real problems instead of making political hay.