The metal chosen will determine how conductive the coating is and how well it will stand up to oxidation and corrosion. Flakes are generally used rather than powders. Flakes give higher conductivity, because of their great surface area. Similarly, a big flake will give you better conductivity than a small flake. The most common pigments used are branched carbon, nickel flake, and silver flake.
Branched carbon is the form of carbon powder that is the most conductive.
It offers the poorest performance among pigments available, but it also offers the lowest price. Carbon paints are better used for grounding rather than as shielding coatings. Nickel is a much better performer than carbon, and it is still reasonably priced. Nickel is good at standing up to the elements and resisting oxidation as well. In addition, nickel gives protection against the magnetic portion of EMI, whereas the other common pigments only protect against the electrical portion.