Elements with the Highest Reactivity
Fluorine is the most reactive element - a stream of fluorine gas will spontaneously ignite a concrete block at room temperature.
The most chemically reactive metal with a stable isotope is cesium. Cesium reacts explosively with water.
Elements with the Lowest Reactivity
The least reactive metal is platinum. Platinum cannot be oxidized in air, even at high temperatures. It is unaffected by common acids, although it can be dissolved in aqua regia (a mixture of nitric acid and hydrochloric acid in the ratio 1:3) to yield chloroplatinic acid (H2PtCl6).
Gold is the second least reactive metal.