Fluorocarbon Leader Material
Fluorocarbon leader material's main advantage over monofilament is that is even harder for a fish to see as it's refractive index is closer to that then water. How much this benefit matters varies depending on when and what you're targeting. Many people feel it makes no difference at night, for example. And if fluorocarbon is all scuffed up it certainly can't hide as well as it normally would either.
But if you're looking to be completely sure you're stacking the odds in your favor as much as possible, especially when it comes to fish with sharp eyesight, fluorocarbon is always a good call.