What’s best, or does it matter at all?
Are you talking about painting the wheels sets on rolling stock or locomotives? If it’s rolling stock I have used a varity of paint but mostly Flowquil Poly scale acrylic paint. I personaly do not paint the wheels on my locmotives although a lot of guys do. I have used stuff such as blackening solution & Neo-lube as they both conduct elctricity. I’ve seen where one guy has a rig set up so he can run his steam locomotives in place as he sprays the drivers with an airbrush so as to get complete coverage a very slick idea.