To my knowledge, silver is the best conductor of both heat and electricity among metals with a thermal conduction value of about 430 W/(mK). Gold and copper both come respectably close to silver, and with Copper being significantly less costly it is often chosen over silver in many applications.
Considering this, which material is a good conductor of heat?
Obviously, string is an insulator, and copper is a conductor. As a general rule, substances which are good conductors of heat are also good conductors of electricity. Thus, all metals are conductors, whereas air, (pure) water, plastics, glasses, and ceramics are insulators.
What materials are not good conductors of heat?
Basically, when it comes to conducting heat, not all substances are created equal. Metals and stone are considered good conductors since they can speedily transfer heat, whereas materials like wood, paper, air, and cloth are poor conductors of heat.