Maiden, no contest.
I love Ride the Lightning more than a good chunk of Maiden's albums, and of course their other three albums from their "golden era" are great.
But frankly, even Maiden's allegedly worst albums contain some great songs, and their allegedly worst moments--"The Angel and the Gambler" among them--have a certain draw to them. I can't quite put it into words, but there is something damn special about their music, something that strikes me as meaningful and personal. No, that's not objective, but if they're getting an emotional response from me, they must be doing something right.
Also, I know this is controversial, but Number of the Beast and A Matter of Life and Death are actually among of my LEAST favorite Maiden albums, and The Final Frontier, in addition to being my favorite reunion albums, is easily in my top five Maiden albums overall. I love The X Factor too, though my favorites are easily Seventh Son of a Seventh Son and Somewhere in Time.
The Number of the Beast was a good introduction for Bruce, but Run to the Hills, for example, has never been all that interesting to me. While there aren't many bad songs on the album, the only two phenomenal ones are TNotB and Hallowed Be Thy Name. Mind you, the latter is amazing, but that doesn't mean the whole album is.
Whatever. It's still a great album, and even my two least favorites (NPftD and FotD) have their moments.