I hate to admit it, but for the moment, probably the safest route to go is natural gas.
There is no real difference between Oil, Coal, and Natrual Gas. They are all hydrogen atoms bonded to carbon attoms. The difference is in the ratio of carbon to hydrogen: Coal has more carbon atoms per hydrogen atom, Oil has roughly twice as many hyrogen atoms, and natural gas has at least four times as many hydrogen atoms than carbon atoms.
If it isn't obvious, the power we get from burning coal, oil, and natural gas does not come from burning the carbon, it comes from burning the hydrogen. The most efficient and cleanest burning fuel is Hydrogen, but it is not efficient to extract it from water. Fortunately, we have many gigawatts raining down from the heavens at any given moment that we've yet to put to any useful work.
You're forgetting one thing: Coal, oil and natural gass arn't purely carbon compounds.
Natural gass is nearly pure light carbon compounds, with a light amount of sulfuric and nitric compounds added in (this amount is usualy neglegible)
Oil is meium and heavy carbon compounds mixed with a nice amount of sulfur containing compounds and maybe some nitrogen compounds.
Coal's a mix of pure carbon types, carbon compounds, sulfur compounds and heavy metals.
So yeah, there's a big difference between coal and natural gas, since when you burnt he coall you get a LOT of side products that are usseles, toxic and generaly anoying. When burning gas you get pretty much only water and carbon dioxide (and maybe a bit of sulphuric oxide but that's easely removed with a gas scrubber)
As for the different types of carbon, Graphite, amorphous carbon and diamond are all carbon but their cristal latices differ quite a bit (Complete chaos for amorphous carbon, sheet like for graphite and fused pyramids for diamond) and this gives them wildly different properties.