Market forces cannot determine the "most efficient and clean" energy sources unless the cost of "dirty" is born by those using the fuel. At present, it is not. Instead, when CO2 is released into the atmosphere, the general public eats the cost.
The "most efficient" gasoline is leaded gasoline, if you don't count the cost to future generations of brain damage to children. And, absent non-market intervention, that cost was historically not counted.
There must be some mechanism, such as a carbon tax, to ensure that the cost to future generations of the climate change that CO2 emissions drive is counted. It has to be part of the price of coal and oil. And then, we can see which fuel is in fact, and all things considered, most efficient.