The resistivity of metals and metal alloys is a fascinating study in its own right. Generally, the resistance occurs because drifting electrons scatter: the scattering centers can be crystal lattice defects, "minority" atoms in the alloy, and a host of other things. That is why the lowest resistivity (highest conductivity) usually is found in a pure single crystal of a single element. For practical use, OFHC (oxygen-free high conductivity) and OFE copper gives the highest conductivity of commercially-available copper materials. (There are other oxygen-free types, but OFHC achieves this by vacuum melting, while phosphorous-deoxydized copper leaves holes in the material where the oxygen used to be). Legitimate uses of OFHC (not golden-ear speaker cables) usually exploit another feature: resistance to metal fatigue from repeated bending. The informal test is to clamp a flat piece, maybe 3 mm thick, in a vise and bend it back and forth with a pliers and see how many bends it takes to break it. ETP copper, the normal form for electrical wires, etc., is <0.04% oxygen, but will fail the bending test before OFHC.