Best item

[Endoperez]

Effects don't stack with themselves, and duplicate items don't stack their special effects (unless you abuse a bug). So two identical Staves of Elemental Mastery won't produce +2 to elements, but different ones will.

As for finding which items give identical effect with each other or a spell, check the manual. Marble Armor and Boots of Stone both have the effect "Stoneskin", so that won't stack with Stoneskin the spell.