Coal.
That depends a lot on what you mean by "this source".
fossel fule
The most common sources of electricity generation in the United States are natural gas, coal, nuclear power, and renewable energy sources like hydroelectric, wind, and solar power. Natural gas has become the largest source of electricity generation in recent years due to its low cost and abundance.
Coal
In the US, nuclear power provides about 20% of the total electricity generation, making it the largest source of low-carbon electricity. There are 93 commercial nuclear reactors operating in 28 states.
Figures for 2006 showed coal at 49 percent, and both nuclear and natural gas at about 19 percent.
Currently around 71%: About 49% of electricity is generated from coal, 21% from natural gas, and 1% from petroleum. Adding those together, you get about 71%.
it is one of the leading ones
Oil
The primary energy sources used to generate electricity in the US include natural gas, coal, nuclear, and renewables like hydroelectric, wind, and solar power. Each source contributes a varying percentage, but collectively they make up the majority of electricity generation in the country. The shift towards renewable energy sources is increasing, with the aim of reducing reliance on fossil fuels and lowering carbon emissions.
The most abundant energy source in the US is petroleum, which includes crude oil and natural gas liquids. These fossil fuels are widely used in transportation, heating, and electricity generation.
Coal is the most used source for electricity in the US. World wide, other countries, I don't know.