This is a question that many would answer the United States. For years they seemed to lead the world in military, economics and decision making. But in truth and reality alone there is no one person or government that rules the world completely. According to some America is losing its top grip on international power.
Normally if you ask a question like this people have always thought and considered the actors in world affairs are the states, primary the ones with the greatest powers, and most people consider their decisions and the relationships behind them. When examining states each has a complex internal structure where the decisions of the political leaders are heavily influenced by the internal concentration of power.
It appears that western power is starting to be challenged by East Asia. China and Pakistan are hoping to spur industrial development in Pakistan. In the end, these two countries are hoping that Pakistan will finally take a role in helping to control domestic terrorism.
Furthermore, you should take a look at the EU and why it was formed. I do believe in the next few years things will change and the United States will no longer be the leader of world power. I could be wrong, but with the current President in power, the United States is losing a lot of respect from other countries around the world.