y'know, this is a kind of tough one for me. I'm going to say No Prayer for the Dying, because even though none of the songs are truly bad (NotB and Virtual XI are the only two albums I'll acknowledge as having an outright bad song) a lot of this album is truly forgettable, and let's face it that it's very un-Maiden to be forgettable. Fear of the Dark has some forgettable passages as well, but nothing on No Prayer is as memorable as the title track, Fear of the Dark (even if it's a bit played out at this point, it's hard to not acknowledge that it's a classic). Additionally, of the songs that I consider underrated classics from both of these albums, I can tell you that I think Judas Be My Guide is 10x better than Public Enema Number One. But hey, at least they actually played Public Enema Number One live.
That said, I'd say that I only think that No Prayer is my least favorite studio album, most of these songs when played live sound mean as hell, and I love that.
The album whose material is weakest when played live is probably actually reversed to Fear of the Dark for me (I really had to think about that one) because although I liked the stuff that appeared on Raising Hell, ultimately other than FotD itself, the live songs actually played are largely forgettable (YOU COULD RECTIFY THIS IF YOU PLAY JUDAS BE MY GUIDE, GUYS
)
Final Note:
Blaze-era stuff rips, especially live.