I was pondering a similar question recently. The greatest 5 year period for commercial hard rock (imo) was 77-81. The list of massive, genre defining records during that period...is unrivaled. To narrow it down to one year is just too difficult. That said, the biggest commercial hard rock record of all time was released in 1980. Hard to argue with the list of releases that year: Back In Black, British Steel, Blizzard, Strong Arm Of The Law, Iron Maiden, High Infidelity, Ace Of Spades...etc.