This was harder than I thought! 1986 was easily the peak and I could've put any one of a dozen albums in there, but seeing as MoP is the best metal album ever, it's a no brainer.
1980 - Ozzy Osbourne | Blizzard of Ozz
1981 - Def Leppard | High and Dry
1982 - Iron Maiden | The Number of the Beast
1983 - Accept | Balls to the Wall
1984 - Metallica | Ride the Lightning
1985 - Yngwie Malmsteen | Marching Out
1986 - Metallica | Master of Puppets
(although Slayer's Reign in Blood and Megadeth's Peace Sells... also came out this year)
1987 - Testament | The Legacy
1988 - Queensr˙che | Operation: Mindcrime
1989 - Badlands | Badlands