Answer:
At most temperatures, silver has lowest resistivity.
However, superconductors have zero resistivity, which is the lowest. Superconductors are real, they are primarily used in very powerful magnets (such as in CERN). Known superconductors require VERY low temperatures which require expensive technology.
There is ongoing research to find compounds that exhibit room temperature superconductivity. If (when?) one or more is found, major changes to electricity as we know it will result. Not only will we be able to make very powerful magnets (for use in very powerful motors, generators, etc.), but the power companies will be able to move electricity from the generator to the user without loss. Today typical power transmission losses are between 8% and 15%. There may also be important benefits to computer and processing capabilities.
Hope this helps! :D
