Since pure graphite only melts (or sublimates) at extremely high temperatures, then it follows that something like graphite only having a spare handful of hydrogen atoms thrown in could have an arbitrarily high melting point. However I haven’t been able to find a cite for a particular example. I would presume the hydrocarbon with the highest melting point would be either some variant of polyethelene or one of the polycyclic aromatic hydrocarbons. The highest I could find specifics on was Coronene, with a listed melting point of 438C, which isn’t bad but is there a better contender?
I suspect that you will not find a definite answer. Many CH polymers (e.g. polyphenylene) do not melt, just decompose at around 500 C. Above 500 C, carbon-carbon single bonds start to break, and only cyclic species survive. So by making a polycyclic species longer the melting point can be raised indefinitely until decomposition. Not long after 600 C, even aromatic cyclic species will start to react and essentially form carbon.
polyethylene, containing lots of single bonds, will start to break down above 400. If I find anything better than coronene I will let you know
These are the best I could find.
Hexabenzo[bc,ef,hi,kl,no,qr]coronene 460(sublime)
Hexabenzo[a,d,g,j,m,p]coronene 516
You are right that the PAH’s are going to be the highest melting hydrocarbons. Anything else will decompose. I would guess that even these don’t stick around for long at that high a temperature.
Next I’ll look at corannulene derivatives.