Of the three thermodynamic quantities, δg, δh, δs, which is most useful for determining spontaneity of a reaction?

Of the three thermodynamic quantities, δG, δH, δS.

δG is most important in determining spontaneity of a reaction as spontaneity of a reaction can be determined by its sign, when δG is negative, reaction is spontaneous and when δG is positive, reaction is non-spontaneous.

To check the spontaneity of a reaction, following reaction is considered:

δG = δH-TδS

Here,

δG is gibbs free energy

δH is enthalpy change

δS is entropy change

T is the temperature

Based on the sign of δH and δS, δG sign can be determined that tells about the spontaneity of the reaction.