A thermal diode is a heat transfer device that conducts heat more effectively in one direction than in the opposite direction. Here, we report a planar phase-change thermal diode that transports over 100 times more heat in the forward mode compared to the reverse mode. This effective thermal asymmetry is achieved by injecting water into a vapor chamber composed of superhydrophobic and superhydrophilic copper plates separated by an insulating gasket. In forward mode, dropwise condensate continuously returns to the evaporator by spontaneously jumping out-of-plane upon coalescence on the superhydrophobic surface. In reverse mode, filmwise condensate is trapped on the superhydrophilic surface, drying out the evaporator and eliminating phase-change heat transfer. Our diode is unique in achieving a diodicity of two orders of magnitude with a planar configuration that is inherently suitable for 3D applications and completely independent of orientation and gravity.