The two most reactive elements are fluorine and oxygen. These elements that are gases at room temperature have the highest electronegativity according to the Pauling scale. The electronegativity for fluorine is 3.98 while the electronegativity for oxygen is 3.44.
Electronegativity means that the atom lacks an electron and is eager to steal an electron from another atom. These atoms readily form compounds and are rarely seen in a free state.
Though the elements themselves are very reactive and even dangerous to handle, they are able to form compounds so stable as to be inert. For example, when fluorine is bonded with carbon, the result is Teflon.