Thermal diodes, or devices that transport thermal energy asymmetrically, analogous to electrical diodes, hold promise for thermal energy harvesting and conservation, as well as for phononics or information processing. Thermal diodes enabled by phase change materials are ideal candidates for such applications due to their potential to efficiently operate at ambient conditions under low temperature biases (< 20 K). Theory and experiment for high-performance, ambient thermal diodes, enabled by junctions of phase change materials, is presented. It is shown that such diodes possess ideal temperature biases and material dimensions for optimal operation and design, and their performance greatly exceeds that of ambient thermal diodes operating under low temperature biases in the literature to date. In addition, the application of such thermal diodes for ambient energy harvesting and conservation applications is addressed both experimentally and theoretically.