The element that has the highest first ionization energy is helium. The first ionization energy represents the amount of energy needed to make an atom into a positive ion, which is called a cation.
As the number of protons in the nucleus increases, the ionization energy increases as well; therefore, the ionization energy increases from left to right across the periodic table. Another trend with ionization energy is that it increases from the bottom to the top of the periodic table, because the closer to the nucleus the electron orbits, the harder it becomes to rip the electron from its orbital. As it sits at the top right of the periodic table, helium naturally has the highest first ionization energy.