Inert gases
Reason — Ionisation potential (or ionisation energy) is the energy required to remove the outermost electron from an atom in the gaseous state. It generally increases from left to right across a period (because effective nuclear charge increases) and decreases down a group (because atomic size and shielding increase).
Noble gases have completely filled valence shells and very small atomic radii within their periods; therefore, much more energy is needed to remove an electron from them, giving them the maximum ionisation energy among the groups listed.