Caesium is the least electronegative alkali metal as electronegativity decreases as we go down the group. Caesium is a group 1 element and lies down the group.

Electronegativity increases as we move along the period and it decreases as we move down the group. So this states that group one elements are least electronegative elements in a period and the element at the bottom of group 1 will be the least in the group. Thus caesium has the highest electronegativity as it has the largest size due to decrease in the effective nuclear charge.