Sliver is the best if pure. However, as someone stated above silver does not have as good corrosion resistance. Gold is better for that. You would be surprised how much a little corrosian can do to your resistance.
Someone go the stats out of a testbook above, which is good ..But again that is pure. We aren't getting pure products. It is also not measured after some corrsion is present.
Most of the electrons travel on the outside of the conductor...so the plating is one of the most important factors.