It depends on what Pokemon you have. Leftovers are usually used on Pokemon with no recovery, Choiced items are usually for Sweep monsters, Assault Vest for defensive attacker Pokemon, Air balloon for Pokemon that are weak to ground and switch in often, Mega stones well for Pokemon that mega evolve, Toxic orb for posion healers, Chesto Berry for Restochesto set, Sitrus usually for Harvest users, Evoilite for Pre evolutions that have more bulk then final evolution, Focus sash for squishy leads etc.