If you would have asked me last year I would have said Powerslave.
But this year I am gonna have to go with The Number of the Beast.
Where Bruce makes his first appearance. Cmon, run to the hills, children of the damned, number of the fucking beast. This album almost single handedly created and defined an entire genre of metal. All of melodic death, black, scandinavian death, metalcore, owe it all to Iron Maiden.
I dont like later maiden, I forget they exist after Powerslave.