Sad But True is just Harvester of Sorrow but better.
That has nothing to do with the thread topic, I just felt like saying that.
Oh wow I loved Harvester of Sorrow, my first favorite song really (I was 10 when it was released) and I still just love the one time sound of Justice..., production bleakness and all.
Answer to the question is without question the black album, radio and video, and tour. That was an all time giant success that crossed fan bases and genres. For tons of people Metallica *is* metal.