Number 6
Ancient Mariner
That's something that I can understand, but for me, I consider the entire album to be one gigantic build-up for Empire of the Clouds. You've got 10 great tracks, none of them bad, and even a couple epics in there. Everything is already great as far as albums go, and then Empire comes on and you realize that you've been waiting for this entire moment from the start of If Eternity Should Fail. Which, by the way, is what I think all good closers need to be. Even if they aren't the best song on the album, they need to firstly provide closure and secondly a good deal of pay-off. This is why I like Empire, Rime, Fear of the Dark, Como Estais Amigos, Hallowed, and... most of Maiden's album closers.
Good point on Maiden's closing tracks. I 100% agree, and I think Maiden succeed most of the time. For me, there are very few closing tracks which were "disappointing", to some degree (but that's not to say they are bad songs in any way). "Iron Maiden" (it works wonders as a show closer, but for a studio album it just felt underwhelming), "Drifter" and "The Unbeliever" come to mind. "Only the Good Die Young" might be the band's best album closer yet (even though my favorite Maiden song is actually "Rime of the Ancient Mariner").
Well, all in all, "Empire" is a nice closer, too. Since it was ultimately made a part of The Book of Souls' track listing, there wouldn't be another track to fit as a closer. (Maybe, just maybe "The Red and the Black", but while I feel it does belong on the album, it wouldn't be very fulfilling to close the album with it.) It's just that, musically, it's such a drastic departure from the material on the rest of the album that it feels disjointed.