Electropositivity can be defined as the tendency of an atom to donate electrons to form positively charged cations.
In a periodic table, electropositivity decreases along the period (left to right) and increases down the group. As size of atom increases.
Hence, Caesium is the most electropositive of the stable elements.
Note: Francium is more electropositive than Caesium but it is unstable as it is highly radioactive. therefore it is not consider as most electropositive element.