Number of the Beast has some of my favorite tracks played on Live After Death, but for studio efforts it's definitely not my favorite studio album. If forced to choose, I think I'd have to go with Piece of Mind just over Powerslave, with Seventh Son... a very close third, primarily on the strength of it as a concept album, and it being my first Maiden album I bought after Live After Death. Number of the Beast comes in right behind that.
Congrats on getting the black triangle version, though!