When it comes to 'Big 4 of Metal' I would rank them like this, based on my preference:
1. Black Sabbath (first 6 albums plus Heaven & Hell & Mob Rules and maybe I could add Dehumanizer, but maybe!
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2. Iron Maiden (first 7 albums plus Brave New World)
3. Judas Priest (70's run of albums Sad Wings - Unleashed in the East, Screaming for Vengeance and Painkiller)
4. Metallica as the distant fourth (Ride, Master, Justice)