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.
Can't think of anything I'm keeping back. Just trying to make a low resistance coil to detect current induced by magnetic fields. My research so far shows silver is best but not much better than copper. Some of my colleagues think gold is best because it is used for semiconductor components but that is just on contacts.
Your colleagues are making guesses. If you need ultra-low resistance (and I'm still not convinced you do), have you considered superconducting materials? What are you trying to measure? A magnetic field that induces 200mA in a single coil of wire is pretty damn hefty...
