My god, how do I choose....ok I'll make the list:
1. Powerslave (probably the greatest metal album of all time, along with Rust in Peace and Mindcrime)
2. Seventh Son of a Seventh Son (it's overplayed, but still sooo EPIC)
3. Number of the Beast (especially since it has the greatest Maiden song of all time)
4. Piece of Mind (ALTHOUGH the Live After Death versions are significantly better. Let's face it, Revelations live >>> Revelations studio)
5. Iron Maiden
6. Somewhere in Time
7. Killers
8. Dance of Death (even with the worst cover art of all time)
9. Brave New World
10. No Prayer for the Dying
11. The X Factor
12. Fear of the Dark
13. Virtual XI
If they re-did X Factor with Bruce, it could jump to the #8 position, just behind Killers.
EDIT: By the way, the first 7 in that list are practically interchangeable.