Final answer:
Fluorine has the highest first ionization energy among the halogens due to its position in the periodic table and periodic trends that dictate an increase in ionization energy across a period and a decrease down a group.
Step-by-step explanation:
The halogen with the highest first ionization energy is fluorine. Ionization energy tends to increase across a period on the periodic table due to the increasing nuclear charge, which attracts electrons more strongly and makes removing an electron more difficult. Because fluorine is the first element in the halogen group (Group 17) and is at the top of the group, it has fewer electron shells than other halogens and a higher nuclear charge effective on the valence electron, thereby it possesses the highest ionization energy compared to iodine, bromine, and chlorine. Moreover, the outer configuration of all halogens is ns² np⁵, and adding one more electron allows a halide ion to reach a stable noble gas configuration, contributing to a higher ionization energy. Fluorine's high ionization energy is consistent with periodic trends which predict that ionization energy increases across each period from left to right and decreases down each group on the periodic table.