What we view as "filler" and why is obviously a bit subjective. I found this definition to be
close to how I view it:
Album Filler are songs that take a perfunctory, Strictly Formula stance on creation in order to have something distinct to fill in some time. They're usually straightforward, unimaginative, and otherwise forgettable. Of course, it isn't set in stone that a song will suck for being filler.
https://tvtropes.org/pmwiki/pmwiki.php/Main/AlbumFillerThat's not exactly right, at least how I view it. But it's close. I guess, to make it a bit closer to how I view it, it's something like this:
A song is "filler" when, after all is said and done, the band mostly has no aspirations of the song being a "hit" or a "favorite" or a "requested" song. It may never get played live at all, other than maybe when touring in support of the album it appears on, or where the band is intentionally going for rare, deep cuts. It's one that was written, wasn't bad enough to "cut," and left on the album because it's decent and fleshes out the album length.
That's about how I see it. So that said, the songs I consider "filler" on some of the early "classic" Maiden albums would be:
NOTB:
-The Prisoner
-Gangland
[I think Invaders
almost feels like filler. But it's hard to classify an album opener as true "filler" because a band and anyone else working on an album will generally feel that the album opener was distinctly chosen as a strong opening song]
POM:
-Quest for Fire
-Sun and Steel
Maybe Still Life
Powerslave:
-Flash of the Blade
-The Duellists
-Back in the Village
I was also tempted to list Losfer Words (Big 'Orra), but I'm not sure what the general thought is on that song. I'm giving them the benefit of the doubt that they thought it was a cool, novel thing to do an instrumental and that they were happy with it. But I don't like it, AND it feels to me like just something extra to fill space, so I personally would lump it into the "filler" category. But, again, I'm giving them a bit of the benefit of the doubt and just chalking it up to a grand idea that fell flat in the execution rather than something that is more blatantly filler.
SiT:
-The Loneliness of the Long Distance Runner
-Déjà-Vu
Note: I like both of these songs a lot. But they still fit my subjective definition of filler.
[I'm not ranking Seventh Son, even though I recognize that most would consider it part of the "classic" period. I just don't like the album and cannot tell you how more than a couple of songs even go. SELF-EDIT: OK, after looking at the track list, I actually know 4 of the 8 songs. But still, I can't really comment on the other half of the album.]
It's also interesting to note that, for bands that came out of the '70s, '80s, and even up through the mid-'90s, I think there's a noticeable pattern that "filler" songs will generally come in the back half of where "side 1" would be on a cassette or vinyl version of the album, and the middle of side 2. There was a general consensus that that is where you would "hide" the weaker songs to pad out the album. You would usually see a pattern something like this for an 8-10 song album:
-Strong opener, but a song that is perhaps not quite "quality" enough to be a single
-1 or 2 singles
-1 or 2 "filler" songs to round out side 1
-Strong song to lead off side 2 and give people incentive to flip the record/tape over
-Maybe another single
-2-3 filler songs
-Strong closer