Noble gases have the highest ionization energies. Also, the removal of an electron from a stable state (octet or duplet) is very difficult.
Ionization potential or ionisation energy is defined as a measure of the difficulty in removing an electron from an atom or ion or the tendency of an atom or ion to surrender an electron. The loss of electron usually takes place in the ground state of the chemical species.
The noble gas has a shell packed with valence, so it resists the removal of electrons. Noble gases have a complete electronic configuration and have the stability. The removal of electrons is difficult in these cases. Hence they have the highest ionisation potential.