This sounds like a homework question!
If it isn't a homework question then silver is the best conductivity material in wire form - but it does not come cheap and is difficult to obtain except to special order. Unless you are designing something like a near-perfect output transformer for a high-end valve (vacuum tube) audio amplifier, it's not usual to worry about wire losses - especially at a current of only 200mA.
Is there something you are not telling us?