Rhodium is the best material for contact surfaces. It is harder than gold. Rhodium is a good conductor (although this is not important for contacts), and rhodium does not tarnish. Second is gold, since it doesn’t tarnish as much as other materials. All military connectors use gold. Rhodium is too expensive. But gold is soft and not a great conductor compared to copper or silver; so it is generally a poor choice for the wire itself. Silver is the best conductor for wire.
