The thing is, No prayer doesn't have a single song I would call great. Fear of the dark, on the other hand, has Afraid to shoot strangers and the title track. However, the lows on Fear are just extreme lows, easily the worst songs the band has ever put out, such as Weekend warrior, Childhood's end, The Fugitive, The Apparition. No prayer has only one bad song, which is The Assassin. That's why Fear gets my vote, there are waaaay too many songs that drag it down, whereas Prayer has a bunch of okay songs.
Just talking about Maiden album having okay songs instead of fantastic ones reminds me that these two albums are hugely below their standards.