Per NASA document: https://history.nasa.gov/conghand/propelnt.htm
Super high-energy bipropellants_______________________ 300 to 385, ISP Fluorine-Hydrogen Fluorine-Ammonia Ozone-Hydrogen Fluorine-Diborane
But Fl is very difficult to handle. Doing a combustion experiment back in mid 60s, I had a regulator burn up that was apparently not properly passivated that released a whole bottle of flourine to the atmosphere. No injuries, luckily. tom kosvic