I have always liked brass on a knife. I really like the aged mellow look it gets with time. Nickel silver is also a favorite and I have never had much trouble keeping it clean. Copper is one metal that will tarnish and turn green before you know it and it surprises me that people will list it as a favorite guard material, but not brass because it tarnishes. I might use copper as an accent material in combination with other materials but never again by itself.
Much of the guard material choices to me depend on the knife and the uses it will be put to. While I like stainless on a hunter or fighter, I don't care for it on a traditional Bowie with a carbon steel blade. I would much rather use wrought iron, blued steel, or nickel silver.