Good insulators are the ones that do not have free electrons. This prevents flow of charges from one point to another. Some of them are non metals like plastic, rubber, glass and air.
2 Additional Answers
An insulator is a material that does not allow flow of electric charge though it. Glass, paper and Teflon are examples of good insulators. Wood products are also good insulators as well as they contain a lot of air pockets.
It depends on what you are wanting to insulate. If you want to insulate your home, then fiberglass is one of the best insulators. If this is to insulate for electricity, then glass, ceramic, or rubber insulators are usually used.