things that help it in battle.
like a pokemon with high HP would benefit from holding a
Left Overs item.
some other good items are like
King's Rock
Scope Lens
Lax Incense
those are the 3 I generally use in the battle tower
the move type boosters aren't really all that useful.
i.e.
mystic water
hard stone
they're not so great, there's better items you can give a pokemon to hold
berries that cure status are decent hold items too
too bad there's no berry like "miracle berry" in ruby sapphire
or if there is I don't have it 'cuz I don't really use the berries too much
except when making pokeblocks to evolve my feebas.