Four of Maiden's first five albums are truly essential purchases. It's quite amazing how Steve Harris was so focussed he was able to write such classic material in the space of five and a half years (1979-1984). This marks the end of an era, and undoubtedly, Maiden's best, none of the subsequent eight studio albums that have followed "Powerslave" have come close to the watershed of the early years. This isn't the best of the five, but lyrically Messrs Harris, Smith, Murray, McBrain and Dickinson have really pushed the envelope. The title track absolutely kills, with a trippy slowed down break in it. Aces High is another standout track, which improves with each listen. The gruesome subject matter of Two Minutes To Midnight belies the brilliance of this song.
This, however, isn't the best of the Maiden albums, despite being the second one to come to mind after the seminal "Number Of The Beast". "Piece Of Mind", originally released in May of 1983, is my personal favourite, and is often hugely overlooked cos it didn't have the staple songs that Beast had (ie Run To The Hills, Hallowed Be Thy Name, NOTB etc.). But it does have the best songs in my opinion. So start with that. The debut album (Iron Maiden) is probably the next best one to buy, an amazing debut. Killers isn't great and is the weakest of the five.
So, in this order: buy NOTB, Piece of Mind, Iron Maiden, Powerslave and Killers. Buy them now!