All depends on what the application is. Do you need mechanical strength as well and what your definition of the "best" insulator is? My guess would be a ceramic based material or glass.

Regardless of the other properties, I would say PTFE (teflon) or PI (kapton).

By " insulating material" you mean electrical or thermal insulating material? Polymers or plastics are the two in the same time. In ceramic materials you can find Electrical insulating and thermal conducting (ex: alumina).

For electrical insulations and high thermal conductivity materials, i would offer to you alumina nano filler.

follow

For thermal--mineral and rockwool

For electical-fiber glass, PTFE

It depends on what the application is.

One of the most common insulation materials, fiberglass is used in a wide array of every day applications. Even though CMS wool is marginally more expensive than fiberglass, it is odorless and can withstand temperatures up to 2192º F. CMS wool is used in a broad range of common applications.

https://www.firwin.com/high-temperature-thermal-insulation-product/

Best regards

For more information:

INSULATION MATERIALS AND PROPERTIES

http://tiac.ca/wp-content/uploads/2015/12/TIAC_Guide_English_2013-Section-02.pdf

Best regards