Question

For which gases does the ratio of PV to RT equal a constant?

Answer 1

Presumably, all gases.

By definition, `(PV)/(RT) = Z` is the compressibility factor. When `Z > 1`, the gas's repulsive forces dominate (it takes up more molar volume than its ideal counterpart), and when `Z < 1`, the gas's attractive forces dominate (it takes up less volume than its ideal counterpart).

For any given gas, it is compressible to a given extent determined by its identity. As far as I know, this extent does not change for a given set of conditions (pressure, volume, quantity of gas, temperature, etc) in which we observe a given gas.