How to compare the Heat of Combustion qualitatively?
For isomers differences in the heat of combustion are due to differences in the internal energy of the educts. If intermolecular interaction (lattice energy etc.) can be neglected it reflects differences in the internal energy of the molecules.

In your first example C has the smallest heat of combustion since it has the lowest internal Van der Waals strain.

In the second example the order is given by the ring strain, and the Van der Waals strain between the two methyl groups at the cyclobutane ring.