The most flammable thing on Earth is Chlorine Trifluoride (ClF3).
Chlorine Trifluoride (ClF3) is cited often as being the most intensely reactive and dangerous, and is able to ignite materials like concrete and sand without an external ignition source.
And the element Hydrogen (H2) is the most combustible element, which ignites very easily with a wide explosive range.
Other extremely flammable substances include white phosphorus and certain organometallic compounds, like diethyl zinc that spontaneously combusts in air and is known as being pyrophoric.
Chlorine Trifluoride (CIF3) contains both fuel "chlorine" and oxidizer "fluorine" within the molecule, allowing it to burn incredibly fiercely, even reacting with concrete and sand to produce hydrofluoric acid.
Hydrogen H2 as the lightest element, reacts exothermically with oxygen, and has a low ignition energy and a very wide flammable range in air, which makes it a very powerful fuel.
White Phosphorus P4 also reacts very aggressively with oxygen in the air, and ignites at low temperatures and produces a blindingly bright fire.
And Pyrophoric compounds like Diethyl zinc are substances that ignite spontaneously upon contact with air because they are highly reactive with oxygen, requiring no spark or flame to ignite.
Other highly flammable things are acetone, gasoline, propane, diethyl ether, methanol, turpentine, acetylene, natural gas and even carbon monoxide.