China, with its 1.3 billion citizens, is more pollutive than any other nation by a wide margin. Goldman Sachs notes that China had 8321 million metric tons of carbon dioxide emissions in 2010, followed by the U.S. with 5610 metric tons. A large drop-off later is India at 1696 metric tons.
China is not as oil reliant as the U.S., getting the majority of its energy from coal. Saudi Arabia gets most of its energy from oil, naturally, while Brazil has been more effective than anyone in utilizing hydroelectric energy.
Although the main polluter today, China holds the greatest capacity to switch to renewable energy, with 23% of the global capacity. The U.S. owns a significant portion of the capacity as well with 16%, but it is Germany, whose population is only 1/4 the size of the U.S. and 1/16 the size of China, that is setting itself as an energy leader with currently 11% of the total global renewable energy capacity.
Lest you forget the importance of energy efficiency in a developed functioning society, note that 88% of global GHG emissions are not related to transportation.