Why is Flourine the most electronegative element ?

Well, one of them must be, and it turns out that fluorine is the one that has the highest concentration of positive charge and still has space for an electron very near that region of positive charge.  The next electron will be drawn very strongly into close proximity to that positive charge.  That means that such an electron will have to expend a lot of energy to escape, so inversely, it means that the electron releases a lot energy when brought in to that position.  There are also other factors that come into play, such as the ratio of positive to negative (dilution effect) and the quantum effects (shell filling).