United States of America
The United States currently has stronger economy (and much, much stronger per-capita economy and non-modernization growth potential), a world-spanning alliance system (as well as a rather powerful military that would crush any other organized military), and a political system that isn't, impossibly, based on providing endless economic growth.
I go to China a few times a year. It is a polluted mess. They have 65 million empty condo's that were bought with all the peoples savings. They live as family units 10 or more to a home. This market will crash and they will go to war to keep the people from uprising. Their economy will collapse, probably right after ours.
So many more human violation rights
China is probably 5-10 years away from their own economic struggle. After that, give them another 5 years, and they will likely have a violent revolution against their Communist setup. They are trying to run their businesses in the same way that US runs ours, but their political and economic structure is not conducive. And they won't go to war with us, because they know we can outgun them, and then reclaim all our debt, declaring ourselves debt-free. Their only hope, really, is that we can prop up our economy soon enough to help stabilize theirs.
I believe in our military. Our military is pretty strict and very organized too. I think we would win. Its a good match up.
Most Americans are fat. And many of us Americans have forgotten what true patriotism is. Which is sticking by your country no matter what. We don't do that anymore. And china 2 billion population and most people there aren't overweight. So yeah, china steam rolls us