The highest melting point Inspired by this question, what is the substance or compound with the highest melting point? At standard atmospheric pressure and ever-increasing temperature which substance a) eventually becomes liquid b) does so at the highest temperature?

I don't know much about glasses or related semisolids, so perhaps a reference viscosity? Glass transition temperatures are kind of like melting points, but some materials have a glass transition temperature and a melting point.

Some initial casting about on the internet finds that rhenium and tungsten have melting points at about 3400 °C, and hafnium carbide claims to be 'one of the highest' at 4100 kelvin. Does it actually melt or just decompose and then sublimate? 4100 kelvin is its 'refractory temperature', but I'm not certain how that's defined.
Well, according to this Wikipedia article, it might be an alloy of tantalum carbide and hafnium carbide, specifically $\ce{Ta4HfC5}$.  A melting point of 4215 °C (4488 K) is listed.  But who really knows the answer to your question with so many new, proprietary alloys with complex compositions being fabricated these days.