Invader didn't vote when he started the topic. Shame on you, Invader!
I didn't vote when I started the poll because I thought it was a pretty difficult choice and I didn't want to influence the first poster in the direction of any particular choice (which can happen even subconsciously). So I decided to hold my vote. Of course, I could say I forgot, but that would be a boring reason.
Anyway, for me it's an almost impossible choice between Phantom of the Opera, The Trooper, Dance of Death, and The Longest Day. Seventh Son, The Number of the Beast, and Childhood's End get honourary mentions, but I just feel they don't quite reach to the standards set by the others.
Phantom of the Opera has absolutely brilliant instrumentals that are only passed by a select handful like Hallowed Be Thy Name and Sign of the Cross. Its quality is pulled down by the singing, which, even with Bruce, doesn't just seem anything special. No, this song is all about the instrumentals.
The Trooper was once my favourite Maiden song, and even now is among the top. It might be overplayed, but no other song gives me the same surge of adrenaline as this. It is amazing because of the sheer raw power of the song, and for being so great live.
Dance of Death is Janick's songwriting at its best, and tells a story very well. It is a complex song, epic, and it gives vivid images, an ability which is matched by few other songs. And there just is that magic unknown ingredient that makes a song good.
The Longest Day is a brilliant war epic that, in my opinion, matches Paschendale's mood and emotion. The two could almost be considered partners. If only Maiden made a third war epic on their next album, then I could consider them a trilogy. The Longest Day also gives images well, and it has great singing by Bruce. And again, it just has that "thing".
In the end, I'll take the "commercial" route and say The Trooper, because it is the only non-epic out of the bunch. The three epics are too hard to compare.