I wouldn't say that China is the most powerful, simply because (unlike the U.S. and a few other nations) most of their strengths are not yet aligned into a forceful single entity — or perceived entity. Their manufacturing, their use of energy and resources, and their massive population are significant forces in the world, but they have not yet coalesced their political will(s), social culture(s), and ethnic and national identity(ies). If and when they do, they will truly become the most powerful nation on the planet.
However, running counter to the trend of any single nation's dominance is the rise in dominance of the collective will of the scattered peoples of the world. Globalization, the spread of information, cultural interconnectedness, and concerns for such varied issues as sustainability, Earth's environments, human rights, democracy, quality of life, standards of living, religious freedom, etc., are increasingly helping to unite people across national boundaries. Though I may never see it, I look forward to the day when being an Earthling is more important than being Russian, Sri Lankan, Czech, American, Portuguese or Chinese.