Parameters that determines the thermal stability of DNA
Although taken in isolation GC bonds should be more stable than AT bonds, due to their higher binding energy (three hydrogen bodns versus two), it is the stacking energy that is considered the stabilizing factor of the DNA:

We find that temperature and salt dependences of the stacking term
fully determine the temperature and the salt dependence of DNA
stability parameters. For all temperatures and salt concentrations
employed in present study, base-stacking is the main stabilizing
factor in the DNA double helix. A•T pairing is always destabilizing
and G•C pairing contributes almost no stabilization. Base-stacking
interaction dominates not only in the duplex overall stability but
also significantly contributes into the dependence of the duplex
stability on its sequence.

The reason for that is that one can have the same number of pairs under many different configurations of strands, which means high entropy and instability, unless there are stacking interactions. This is why most models of DNA and RNA helices are based on adding the stacking energies, although there are some additional contributions, particularly in the case of RNA - initiation energy, energies associated to the inner loops, etc.