Use these charts as reference for realistic settings:

There are also hints on how to make realistic materials in these charts. In essence it is about choosing a workflow (default or metallic) and obtaining relevant values for your maps or colour pickers. For instance, if we wanted to make shiny white plastic, we would want a white Albedo. Since it is not a metal we would want a dark Specular (or a very low Metallic value) and finally a very high Smoothness.