What is the reactant compound in all combustion reactions?

The reactant compound is any hydrocarbon, e.g. Alkanes, Alkenes, and Alkynes.

With a suitable activation energy , the above substances will react with oxygen(air) to form water vapour and carbon dioxide, and release useful heat energy in the reaction.

A general reaction eq'n is

Hydrocarbon + oxygen ==E(a)==> Water(vapour) + Carbon dioxide ( and heat energy).

If you mix a hydrocarbon and oxygen together, nothing happens. So you strike a match (heat (Activation (energy (E(a))), to set the reaction going. Then it is 'self-serving; in that the released heat energy keeps the reaction going, until either/or the hydrocarbon or oxygen has run out.