Well, in terms of conductivity, the best you can get is pure silver. However, silver is only about 8% more conductive than copper, and when it comes to audio signals, this amount isn't significant. Add to that the fact that silver doesn't take repeated flexing very well, the fact that it is much more expensive than copper, and that it isn't available as pure as copper, and it really rules itself out.
Copper is the conductor of choice because of its high conductivity and its ability to take repeated flexing. It's also widely available in high purity and at low cost.
Gold is actually a worse conductor than copper or silver. It's barely more conductive than aluminum. Gold is used primarily to plate the connectors on terminated cables because of its properties as a noble metal, meaning that it doesn't oxidize like copper and silver do. It doesn't do anything for conductivity, but rather maintains its conductivity over a long period of time so your connection remains good and solid. Besides, the coating of gold on a connector or binding post is so thin that it isn't anything to worry about.
