Which set of chemicals, reacting together, produces the highest amount of energy, per unit of mass of the substrates?
Let's limit the substrates to reasonably stable chemicals (nothing like monoatomic oxygen gas or neutronium; nitroglycerine level unstable is okay.) and reasonable set of physical conditions (not metallic hydrogen, or bose-einstein condensate, although a primary explosive to initiate the reaction, or liquid hydrogen temperatures are ok). Also - a pure chemical reaction. No nuclear fusion or anti-matter annihilation please :)
The question is related to "Is there a maximum Isp for “exothermic chemical reaction rockets”?" - a theoretical top limit will be the kinetic energy of unit of propellant being equal to chemical energy of unit of fuel+oxidizer. Popular $O_2 + 2 H_2$ is chosen for high specific energy, abundance and not being too terrible in handling, but I know there are reactions of higher specific energy, if not too practical in rocketry.