There is no simple and straightforward answer to that question, but since this is a site about legal matters, perhaps a good starting point is to think about who has the most legal power in the world.
In the United States, the Justices of the Supreme Court arguably wield the most legal power. The Supreme Court is comprised of the Chief Justice of the United States and eight Associate Justices, and they are appointed for life (their tenure ends only upon death, resignation, retirement or conviction).
The Supreme Courts of each respective country hold tremendous power in deciding cases and shaping the legal environment of their nation. A list of the Supreme Court cases of 129 countries can be accessed through the Global Courts site.
On the international level, among the most powerful legal bodies is the International Court of Justice, whose function is to settle legal disputes submitted to it by nations, as well as provide advisory opinions on legal questions referred to it.