All the Iron Maiden albums released between 1982 and 1988 are the cream of the crop, but if I were to choose just one, it would be Powerslave, especially with their 13-minute epic "Rime of the Ancient Mariner".
Over the years, I've become a bigger fan of Iron Maiden after making friends with a tribute band based in L.A.
The Iron Maidens (World's Only Female Tribute to Iron Maiden)
There's certainly a difference between Heavy Metal and Hair Metal. Hair Metal relies mainly on image - long hair sprayed with cans of Aqua Net, heavy use of makeup and mascara, spandex pants, etc. Their songs focus mainly on decadence - partying all night and getting laid.
Iron Maiden, on the other hand, never did songs about mile-high clubs and booze. Their songs vary from horror to sci-fi to world history. Just because they wore spandex back in the 80s doesn't mean they were a Hair Metal band.