The most efficient conductors have the lowest resistance. All superconductors are the best conductors. Some are pure metals like mercury and others are alloys such niobium-titanium and germanium-niobium. (Note: Carbon, as M NandaGopal has mentioned, is not a metal and is only semi-metallic in its graphene form.
At specific cold temperatures, superconductors are created from the metals I have listed. Mercury becomes superconductor at 4.2K. However, “room temperature” was not specified in this question.