protection from magic clearly rules here..but i have seen an item, that will give your party incredible strength if you have a bard..ravel's harp of pandemonium..the blowback/confuse ability is awesome..disrupt spellcasters..blow back hordes of enemies so they become sitting ducks for spells
other good mention are the book of infinite spells...pick the ability of your like..true seeing being a potent one
and last, the efreeti bottle..summoning a powerfull efreetti that can cast a good mix of spells