I actually like their first album most for some reason. I feel like it has the most energy and excitement, even if it is raw. Seventh Son of a Seventh Son is probably their 'masterpiece'; it is kind of the culmination of all their years. Even their 2000 and 2003 albums after reuniting were great (Brave New World, Dance of Death).
Charlotte the Harlot is actually by far my favorite song by them, and I must be the only person in the world it seems. Just checking their last.fm, it is only the 106th most popular Iron Maiden song there (though surely some live songs that are repeats are interspersed above that). I do not understand why this is the song off their debut that is ignored when Running Free and Prowler and Remember Tomorrow are still vastly popular, but, as my guess, the reason is probably that the others are much simpler to process!