....non-weapon or armor item in the game. Which is it and why?
I think it's the ring of Gaxx. Many bonuses and it can be combined with magic armors.

I think for armor definetly Red Dragon Scale Since it can be combined with Rings, Amulets and Cloaks of Protection. (add the Ring of Protection +2 and you have the best armor possible in the game.. -3 AC(!)

Staff of the magi... technicall a weapon though since you don't use it as such I think it is allowed... if not then ring of gaxx, possibly amulet of power...

My fav is the boots of speed

One of the lowest AC you can get is -11 but there are even lower ones.

SPOILER
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In TOB, the red dragon scale can't have protective rings with it. So if u have that, don't take it off, or u better have a damn good reason

best item? for sheer mage-destroying fun in a multi-play duel, id have to say the protection from magic scroll. imprisonment? maze? finger of death? laugh.

very funny andrewas. But I'd say Crom Faeyr and Carsormyr. Robes of Vecna are good to.