I'd say No Prayer for the Dying. I mean, apart from Holy Smoke and Bring Your Daughter to the Slaughter, there are many great songs here. Tailgunner, the title track, and two amazing but overlooked tracks, Fates Warning and Public Enema #1, Mother Russia, The Assassin and Hooks In You (the last song penned by Adrian Smith before his departure). Only problem I have with this album is Bruce's growl.
Fear of the Dark is a close, very close second, and earns that position because of songs like From Here to Eternity, The Apparition, Chains of Misery and Weekend Warrior, all of which I find way below par. Many great songs, but many filler songs too.
The Blaze era has to be Maiden's worst period, even though I recently repurchased X-Factor out of curiosity (I had previously bought it, upon it's release, and could'nt get into it and sold it) and think that, musically, it is really strong... But how could Harris and company accept what Bayley delivered ? I mean, he was flat so often ! Never mind Dickinson before him, Bayley was bad (for Iron Maiden). Flat (or sharp) repetitive and unimaginative.
Virtual XI has the same problem (Bayley) but again, musically, it is a good album. I enjoy tracks like Futureal, Lightning Strikes Twice, The Clansman, When Two Worlds Collide and Como Estais Amigo. The Harris epics, on this album, don't work. The Angel and the Gambler, Don't Look to the Eyes of a Stranger and The Educated Fool would have worked better as short songs, but they are long, boring and too repetitive.