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.