Answer:
Ideally you would want something that is lightweight and corrosion resistant. I suppose titanium wool would be best, but I don't know if it exists. Stainless steel wool would be better than regular steel wool because it would last longer. Some of the best suppressors don't use steel wool, but use baffles instead. Some companies use copper wool, but I suspect that it is not as good as stainless steel. You could try using aluminum turnings from a lathe, but I don't know how long they would last.