Among solid, liquid, and gaseous fuels, gaseous fuels are often considered excellent. They mix more easily and completely with air (oxidizer), leading to more efficient and controlled combustion with minimal smoke and soot. This results in higher heat release per unit mass and fewer unburnt residues compared to solid or liquid fuels.

The combustion reaction for a hydrocarbon fuel like methane (a gas) is: $\mathrm{CH}4+2O2\to \mathrm{CO}2+2H2O$

Final Answer: the gaseous fuel