In terms of the military and the economy, the United States is by far the most powerful. In terms of leadership, the US is struggling badly.
Of course most of Obama's moves are political. But, his deeper and darker political and economic agenda supports and forwards class warfare in this country. Make people dependent on a centralized government and demonize those who create innovation and jobs. That is where the current administration is heading.
As of right now I think it would have to be Russia and Putin. He basically took Crimea and has made the rest of the worlds powers look powerless. To the point that President Obama has tied nothing more than to shame Putin and going so far yesterday to go to a college in Europe and talk about immigrants and homosexual rights when he was supposed to be talking about border integrity and national standards of not attacking other nations. Putin has put the US President into a spot where he can't even talk about the issue but went 2008 campaign mode. Now, that is a powerful leader when you box a leader into a hole so much he can't even talk about the topic.
Doubtful, China would send their economy in the dumps as well considering they over half of their exports are going to the US. More so, what if they say give us our money? If we say no nothing will happen. It is much more complex