Here's a level-headed,friendly advice for you,CC007! I got the feeling that you're new to Iron Maiden itself,and you're trying to find those albums which contains the songs that our beloved ladies plays most frequently in their set,so you're not gonna be totally unfamiliar with the songs on the show.If that's the case,then you don't have to spend a shitload of money to get the individual albums,just buy the "Essential Iron Maiden" compilation CD,and it'll have at least 75% of those songs.
But if you really do want to get familiar with all of their albums,here's a quick review to know what you can expect.Depending on your personal taste,if you want to hear:
-classic old school metal with a screaming singer and uncompromised metal sound,get TNOTB,Piece of mind and Powerslave.
-classic old school metal with a slightly punk-ish style singing,get Iron Maiden and Killers.
-progressive rock/metal with wonderful melodies,lots of keyboards and a screaming singer,get 7th son,Brave new world,Dance of death,Matter of life and death,and Final frontier.
-progressive rock/metal with a dark sound,lots of keyboards and a mediocre,depressing singer,get the X-factor and Virtual XI.
-classic metal with weird 80's sci-fi influenced sound,guitar synths,and a screaming singer,get Somewhere in time.
-almost american style hard rock/metal with a screaming singer,get No prayer for the dying and Fear of the dark.
All Maiden albums are good on their own,but as you can see,there's quite a variety between their sound.Choose what you like the most first,it'll make a much better experience.