With the rate China has been going the past 30 years, it should be able to surpass the United States somewhere around the early twenties of this century.
Although China's growth rate has taken a downturn, it has not done without an economic crisis in the United Sates, so I'm fairly sure the IMF, worldbank and the CIA will put China on top of the list on the year 2025.
My reasons given:
First of all, it's population size, the country only needs have around 22-25% per capita GDP of that of the United States.
Second, China is still in it's industrialization period.
There's is still a lot of room for growth.
Third, current US account balance, public debt and government debt will take years to repair, so high US growth rates in the near future is nigh impossible. Meanwhile, China has little public debt, holds large amounts of U.S. bonds and exports far more than it imports.
Fourth, peak oil. After the iraq invasion in 2003, it has become increasingly difficult for nations to increase world oil production, let alone built up a large enough buffer of oil production capacity in case of wars or disaster. Because of this, the price of oil rose tenfold from an asian crisis in 1998 to the beginning of the credit crisis in 2008.
Any large oil consuming country that wants to significantly increase their GDP (and thus oil consumption) now will have to deal with a growth ceiling. As soon as consumption of that country goes up, so will the price of oil.
This makes it harder for nations in debt to spend their way out of it.
In order to still grow, either the extra consumption will have to occur in a transition period for a new world standard of fuel or the nation in question will have to grow at the expense of others, the others being the ones in high debt. United States currently has very large debts and imports more oil than China.
Fifth, other than the United States, there aren't really any other contenders.
Challenge Rogier M Sluimers to a bet on this prediction!