Tough one for me. I agree with many here about Fear of the Dark possibly having higher highs and I could argue that its highlights and, weak as they might be, some songs with more experimental have a lot more character than most of the No Prayer for Dying material.
However, No Prayer for the Dying is just a lot more fun record to listen. Fear of the Dark might have a handful of more refined songs, but...
Looking at it like this: which album is more successful at what it is essentially (or assumably) trying to be? No Prayer might be one of the weaker Maiden albums in terms of general quality and musical ambition, but it's rather good-spirited, energetic and a very rock out kind of a Maiden album that you can acknowledge being remarkably behind the quality of its predecessors and (arguably) most successors, but enjoy the ride with a smiling smile nonetheless. While it probably didn't turn out quite as coherent and intense ride as they originally intended, partly due to lackluster production values, it has certain inspired air to it nonetheless.
Fear of the Dark, however, doesn't feel nearly as sincere. It might be more ambitious for sure, but also a lot more calculated and somewhat heartless at times...
I'm being very egoistic now and quoting myself from the Fear of the Dark album judgement thread:
"The younger brother of No Prayer for the Dying is much more ambitious and has some special talents the big brother lacks, but is nowhere near as honest and good-spirited."