Recommend: Number of the Beast, Somewhere in Time, Seventh Son of a Seventh Son, Dance of Death
Anti-recommend: The Blaze albums. While I like X-Factor (Virtual XI sucks though), I can't recommend it to Maiden n00bs right off the bat.
Recommend: Screaming for Vengeance, Defenders of the Faith, Painkiller
Anti-recommend: The Tim Owens albums, studio or live. He was horribly misused during his time there, and the guitarists drop-tuning their classic songs at concerts was blasphemy.
Given the extensiveness and variety of their discography, it's hard to figure where you might start, but one major thing IMO is that the live albums are far superior to the studio outings, so find the live CDs, especially Absent Lovers, Night Watch and Vroom Vroom