The ideal material for making a mirror would be Unobtainium, which has a zero coefficient of thermal expansion (no distortion as the temperature changes), an infinite elastic modulus (no deflection under load), is able to be machined to precision of a small fraction of a nanometre, and can be polished to make a highly reflective surface.
In the absence of a good supply of Unobtainium, Pyrex or other glasses are about the best mirror-making materials available to us. The great reflecting telescopes of the 1700s and 1800s typically had polished "speculum metal" mirrors, but these were quickly superseded once the technology developed to make a metallised glass mirror.
https://en.wikipedia.org/wiki/Specul...and_telescopes