Fluorine (and francium).
Fluorine is the generally the most reactive element in the periodic table, because it is the most electronegative.
Electronegativity is how much an atom attracts electrons. Reactions are just the movement of electrons between atoms' outer shells. If fluorine is the most electrically-attractive element, then it is probably the most reactive, too.
Fluorine is incredibly electronegative because it has a very small diameter (so the nuclear charge is very close to the surface, so it acts much more strongly) and because it has very few electrons between the outer shell and the nucleus, meaning very little gets in the way of new electrons coming in.
There's also francium, which is incredibly reactive for the opposite reason: francium has one valence electron and an incredibly large diameter. This means that the one valence electron can be lost easily, so this element is very reactive too.