The strength of the wires – strongest to weakest - seem to be in the order of stainless steel, nickel/chrome alloy, brass, silver, copper. The metal you choose will be related to the weight of the piece, the available thickness of wire, and aesthetics.
All of these are subject to fire scale or fire stain, a blackened surface on the wire. This can be removed by abrasive cleaning of the exposed metal. The metal within the glass most often takes up the fire scale too. This can be reduced by thorough cleaning of the metal before enclosing it in the glass. Coating the metal with a flux such as borax often reduces the incidence of the fire scale too.
The techniques of cleaning the fire scale from the metal range from scrubbing and polishing to tumbling. The tumbling has the advantage of hardening the softer metals such as copper, and silver.
Copper looses much of its strength in the firing, and often needs gentle working to stiffen it. This is where tumbling is so useful.
Pure silver normally leaves a yellow stain on the glass. Sterling silver - an alloy of copper and silver – is less inclined to do this. However the exposed wire will stain the shelf and any subsequent glass unless well supported by 1 mm or more of fibre paper.
It is common in silversmithing to pickle silver to remove the fire scale after any heat work.