The thermal conductivity of some of them.

Thallium - 46.1 W/(m·K)

Aluminium -2 37 W/(m·K)

Copper - 401 W/(m·K)

Silver - 429 W/(m·K)

Gold - 318 W/(m·K)

Gold has by far the worlds best eletrical conductivity than any other known material. That would be one heavy heat sink. Id assume silver would do a better job though as it has a higher thermal conductive threshold. Thick silver plated copper heatsink don't sound to bad IMO.