Li F, Na F, K F , Rb F, Cs F
The alkali metals are in the group 1. The electropositivity increases down the group, as the radius of the atom increases. So the attraction force decreases on the outer most orbit electrons.
So in Cs F, Cesium is most electropositive. Floride is most electronegative. The difference is maximum. Hence, Cs F is most polar and ionic.
Li F is lease ionic.