I think the following criteria are important here: The song should be played often in concerts, it should be one of the best songs and it has to be recognized outside the community of devoted Iron Maiden supporters. And it shouldn't be too fresh either, a song needs to deserve the status over a long period.
The only song they _always_ play is Iron Maiden. But i vote for Hallowed Be Thy Name, because of the following things:
- It has more "classic Maiden" in it in terms of composition and sound, while Iron Maiden is just another good song
- It is nearly as mandatory as Iron Maiden in the setlist, so I think it can compete in that matter.
- It is one of the songs that I would guess most non-fans have heard (together with Run to the hills, The Trooper, Fear of the Dark)
- Some of the songs in the list are great songs but are far too new. Fear of the Dark came 10 years after Hallowed ... entered the set lists. Fear ... has of course been a live favourite since then, maybe even more than Hallowed ..., but I think history should matter here.
Rime of the Ancient Mariner is just played too rarely live for it to win this, although I personally rate it higher than Hallowed Be Thy Name (as one of few, Hallowed is high on my list).
So, if any song should be regarded as Maiden's anthem I would say Hallowed Be Thy Name is the choice.