While this shield certainly is good, I wouldn't say that it's the "most" beneficial item there is. It's invaluable against the beholders and such, but ONLY against those. It has little to no use anywhere else, with anyone/anything else. Still, I keep it in my pack..
As to what is THE most useful item, I don't know. Many things to many classes. Bag of Holding definately strikes my mind, very handy, sad that cannot get it right away (BG2: SoA). What else.. Full platemail as such for tank classes, good armor which will still let you use other magical items like rings and such. Crom Faeyr for those hamemr wielders, smacks 'em all and everywhere, not to mention the zoomy bonuses. Carsomyr for 'em goody-goody paladins. Robe of Vecna for the casters, literally a 'must have' item. Staff of Magi, definately handy thingie for any travelling caster. Boots of Speed, when you have the need and lack the speed..
And so on. Many things are handy, in different situations, for different classes. I cannot name one item which is THE superior for all, there's something for everyone
