The fuel AVGAS 91/96 UL from Swedish oil company Hjelmco Oil has been selected as the cleanest aviation fuel in the world, by Federal Office of Civil Aviation, Switzerland. "Clean", as in "low emissions", that is. There are other contenders, mostly based on natural gas. If similar technologies could be applied on coal, the economy could get even better.
Another question regarding the sustainability of aviation, is how to make use of renewable energy. Ethanol as an aviation fuel shows some promise. And where natural gas can be used, so can upgraded biogas. (A matter of time in Sweden?)
Hydrogen is not a sustainable alternative however, considering the current state of production technology. Far too much power is needed to produce sufficient quantities of fuel for long range flying.
Please check the comments regarding key difference between the fuels discussed in this post