Halogens have the highest electronegativity values because as you move across to the right of the periodic table, the elements want to attract more electrons and protons to make the configuration more stable. Halogens have the largest, greatest desire to attract one more electron to become a noble gas stable configuration.
Green squares are the halogens http://www.bbc.co.uk/schools/gcsebitesize/science/images/38_modern_periodic_table.jpg |