Natural gas. Of the three non-renewable fossil fuels (coal, oil and natural gas), natural gas (methane) produces the least carbon dioxide and other pollution.
oil
Anthracite is the highest grade of coal that produces the least pollution and most energy per unit. It is also known as stone coal, or black coal.
Sunlight is tied with both Tide and Wind, only Sunlight is much stronger. Harnessing the power Contained within The Ocean Swells seems very promising, amongst others.
None whatsoever. At least, none that isn't also produced by every other form of energy.
Coal takes millions of years to form. We can't make more of it. Once it's gone it's gone. At least until more forms in a few million more years...
Technically the wind is the lest natural resource used.
Hydroelectricity
potatoes
Vermont
It at least has to be a sprout before it produces oxygen.
Antarctica.
the moon and other planets