Tell ya what, Dolphins - ignore all them folks who posted before me. I shall answer your question.
Here's the albums you need, in the order you ought to buy them:
1.
Live After Death. Contains live versions of songs from the first five Maiden albums (although, sadly,
Killers is represented by a single song). This will give you a great overview of their early career. The live versions are almost perfect (the only fault being that Bruce's voice is a little rough on the opening song, "Aces High").
2.
Brave New World. This album, in my opinion, stands out as the best thing they put out between #3 below and
A Matter Of Life And Death. It's the first album with the current six-man, three-guitar lineup and it has many spectacular songs.
3.
Seventh Son Of A Seventh Son. More great songs from the classic 1980s lineup, wrapped around a somewhat loose and confusing concept / story.
4.
The X Factor. One of the two albums with lead singer Blaze Bailey. This album contains some of Maiden's most ambitious and dark material.
These four purchases will give you a sufficient sample of Maiden's music to decide where you want to go from there.
If you like
Live After Death, you may be interested in the studio albums from which that material was drawn - many great songs didn't make the cut for
LAD. These albums are
Iron Maiden,
Killers,
The Number Of The Beast and
Powerslave. (You already have one of the five,
Piece Of Mind.)
If you like
Brave New World, then you'll probably enjoy
Dance Of Death and
Rock In Rio (the latter being another live album).
If you like
Seventh Son, then
Somewhere In Time should be high on your list also.
If you like
The X Factor, then seek out
Virtual XI, the other album with Blaze Bailey.
You mentioned that you have A Real Live One, which means you already have good live version of the best songs from
No Prayer For The Dying and
Fear Of The Dark. I'd leave those two albums till last, as many fans would say they are Maiden's weakest albums.
I hope that clarifies matters. And if Raven threatens you again, you have permission to take away his "Early Days" DVD and break it into many pieces.