Stainless steel is the best because it doesn't break down with the acids created in the smoke. As the acids rise up in the pipe, they condense and form a liquid which runs down the pipe and eats away at metalic and brick liners. Stainless steel resists that damage. As to the materials, you must use a material that is approved by the state and local codes and tested by a national testing agency for the use intended. I would only use approved materials. If you intend on selling your home in the future, you could be liable for replacement and or damages.