For which gases does the ratio of PV to RT equal a constant?
Presumably, all gases.
By definition, $$\dfrac{PV}{RT}=Z$$ 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.