Complementary media, possessing permittivity and permeability with the exact same magnitude but of opposite sign to their counterpart media, can optically cancel out the counterpart media. This renders fascinating applications including perfect lens, illusion optics, invisible gateway, opening up virtual holes in a wall, etc. However, the realization of complementary media generally requires metallic resonating structures, leading to challenging fabrication technology and inevitable material loss. In this work, we demonstrate a type of all-dielectric unidirectional complementary media by using symmetric dielectric multilayers. We find that such symmetric dielectric multilayers can be regarded as effective media with flexible effective permittivity and permeability, which can operate as unidirectional complementary media of almost arbitrary media including metal, air, zero-index media, etc. Based on this interesting property, we show that unidirectional wave transmission through metal films, as well as extremely impedance-mismatched zero-index media, can be greatly enhanced by using the proposed all-dielectric unidirectional complementary media. Our work opens a simple but efficient approach for transmission enhancement, that may find practical applications in transparent electrodes and tunneling WiFi signals through walls of buildings.