Powerslave, it's been my favourite for some time now. I just think that it is a very consistant record, and even the weakest track (Back in the Village IMO) is very listenable. Some of the best guitar work on here too.
SIT is probably their next most consistant album for me, with only one track (Heaven Can Wait) which I sometimes am reluctant to listen to.
NOTB is fantastic throughout... although I cannot find a weak point on it, I do prefer the other two a little bit more.
POM is very good, although Sun and Steel and Quest for Fire are very weak for me. Good songs which I enjoy listening to, no doubt, but when we are comparing the very best songs from Maiden they fall way behind.
SSOASS has no track I inherently dislike, but I do find some of them more enjoyable if I listen to it all the way through, one after the other.
The Final Frontier is the best since SSOASS, and very consistant IMO, definately up there with the Golden Era albums (for me). Right now it is probably my 3rd favourite album.
Everything from the eponymous debut to SSOASS is closer to perfect than most bands will come to with just one album in a lifetime, in my humble opinion.