I'm really surprised no one went for this, but I'll have to say Piece of Mind. It was the first Maiden album I bought, but still. As far as most powerful metal album of all time, I would have to say Slayer's Reign in Blood. It's not really practical for most situations, though. Most people think Slayer is ridiculously hard core. So yeah, as far as Maiden, Piece of Mind for sure. Even though "Quest for Fire" is probably the weakest song in Maiden's history lyrically, it's far from their weakest musically, and that's what counts. It's metal, for shit's sake.