Which element has maximum first ionisation potential?

Atoms and ions having inert gas configuration (such as helium), i.e. stable (half-filled) configuration have high ionisation potential, i.e. it is relatively easy to remove an electron from a partially filled valence shell, where $Z_{\text {eff }}$ is lower but it is relatively difficult to remove an electron from an atom or ion that has a filled valence shell, where $Z_{\text {eff }}$ is higher.