We theoretically studied and optimized the thermal rectification of spherical and cylindrical conductive thermal diodes operating with two phase-change materials (PCMs), whose thermal conductivities significantly changes in a narrow interval of temperatures. This is done by deriving simple analytical expressions for the heat flows, temperature profiles and rectification factors of both diodes. It is shown that diode geometry has a significant impact on the heat flows and temperature profiles, but not so much on the thermal diode rectification factor. Optimal rectification factors of 63.5 and 63.2% are obtained for the spherical and cylindrical thermal diodes operating between the terminals of VO2 and polyethylene with a temperature difference of 150 K spanning the metal-insulator transition of both PCMs. These similar rectification factors could be enhanced even more with a phase-change material exhibiting higher contrast thermal conductivity than the ones in the present study. The obtained results can thus be useful to guide the development of PCMs capable of optimizing the rectification of conductive heat flows with different geometries.