I propose the hydrogen cyanide - fluorine rocket.
It has the fabulous potential for a vacuum Isp of up to 380s, with a propellant mix density of 1023 kg/m^3, handily beating methalox mixes. It is non-cryogenic and burns at over 4000K.
Exhaust is 43.3% hydrogen fluoride.
there's probably some material that would work for this, like the sort they use in the inside of the oxidizer turbopump in staged combustion rocket engines