Mercyful Fate -
Don't Break the Oath
I was thinking of picking Dio's
Holy Diver, or maybe an Iron Maiden album, but those are probably ending up here anyway. I don't think Mercyful Fate broke into the mainstream, or gained much popularity outside of metal circles, but this album seems to be known and respected by most metalheads and is on many of the "best metal albums" lists and I think that's good enough.