Which ultra-high temperature ceramics make a good electrical insulator?
Thorium dioxide gets you 3390°C.  Have fun getting it, $$$ and also the thorium is radioactive.  You also risk some nonstoichiometry, due to thorium having variable oxidation states, and that could make the compound somewhat conductive.  Magnesium oxide weighs in at 2800°C; but is more stable, cheaper, nonradioactive and having no variation in stoichiometry.