First album, but Killers is a very close second. Maiden Japan was a bad concert album. A lot of my friends saw them with D'Anno (sadly I never did) and say that wasn't indicative of them with D'Anno. I have a few live clips that confirm this.
The first Maiden album is one of my favorite albums of all time.
I remember hearing it the year it came out. It still mamnages to blow me away to this day.