Which Byzantine incendiary weapon's formula was lost and has never been reproduced?

Greek fire, first used around 672 to break an Arab siege of Constantinople. It was pumped through bronze siphons in the bows of ships, stuck to whatever it touched, and went on burning on the surface of the sea - which is what made it decisive against wooden fleets and terrifying beyond its actual casualty count.

The recipe was a state secret of the most serious kind, restricted to the emperor and one family of technicians, and the Byzantines were warned in an imperial handbook that it had been delivered by an angel and must never be given to foreigners. It worked: the formula was never captured, and it was eventually lost by the people who owned it.

Modern reconstructions get close without settling it. The likely base is crude oil skimmed from surface seeps around the Black Sea, thickened with resin and possibly with quicklime or sulphur, heated and pressurised before discharge. The engineering is arguably the harder part - a pressurised siphon of flaming liquid on a wooden ship is a way to destroy your own fleet - and it is the operating knowledge, more than the ingredients, that appears to have died.