The best insulator is a vacuum. In a vacuum, there is no matter to transfer thermal energy, so heat can only be transferred by radiation. Radiation is a very inefficient way to transfer heat, so a vacuum is a very good insulator.

In practice, it is not always possible to create a perfect vacuum. However, there are a number of materials that can be used as good insulators. These materials include:

* Air: Air is a very good insulator, especially when it is still. This is why double-paned windows are so effective at keeping heat in. The air between the two panes of glass acts as an insulator, preventing heat from escaping.

* Fiberglass: Fiberglass is a type of glass that is made into thin fibers. These fibers trap air, which makes fiberglass a good insulator. Fiberglass is often used in insulation batts and rolls.

* Cellulose: Cellulose is a type of plant fiber that is used in insulation. Cellulose insulation is made from recycled paper and cardboard. It is a good insulator and is also fire-resistant.

* Polystyrene: Polystyrene is a type of plastic that is used in insulation. Polystyrene insulation is lightweight and easy to install. It is also a good insulator.

* Polyurethane: Polyurethane is a type of plastic that is used in insulation. Polyurethane insulation is very effective at keeping heat in. It is also fire-resistant.

The best insulator for a particular application will depend on the specific needs of that application.