Question

What enthalpies of formation @ 25 degrees celsius are assigned value of zero?

Answer 1

The most stable form of an element has its standard enthalpy of formation set to zero.
This applies for example in metals like Mg (s) or nonmetals like carbon (specifically graphite) or diatomic gases like O2 (g) (O3 does not have zero enthalpy of formation because it is less stable than O2).