I'll have to agree with you 1986 is indeed one of the best years ever in music. However, I think that 1988 is a tad better:
Iron Maiden - Seventh Son of a Seventh Son
Metallica - ...And Justice for All
Megadeth - So Far, So Good, So What
Slayer - South of Heaven
Helloween - Keeper of the Seven Keys Part 2.
Four excellent albums by four excellent bands (three are thrash
)in both years. The albums by Maiden and Metallica are better in 1988, but Slayer and Megadeth made better albums in 1986. However, 1988 wins over because Maiden's and Metallica's albums are deep, meaningful excellent albums that are excellent because of being
albums, not just being a grouping of songs, as well as because Maiden and Metallica are better bands than Megadeth and Slayer respectively (though Megadeth is better than Metallica).
And 1988 also has Helloween's best album to date.
Honourary mentions:
1984: Piece of Mind, Ride the Lightning, Defenders of the Faith: Three great albums that are, however, outmatched by their followers or predecessors.
1990: Painkiller, Rust in Peace. The best albums by their respective bands. However, 1990 loses out because there is quite little to support these two.
1995: The X Factor, Burnt Offerings (Iced Earth), The Gallery (Dark Tranquillity), Land of the Free (Gamma Ray). Some of the best albums by the respective bands, and it has great albums from a variety of different metal subgenres. These are all albums that are great to listen to as albums, like 1988. However, they just don't have the legendary status of the 1988 albums.
2002: Rock in Rio, Damage Done (Dark Tranquillity), Versus the World (Amon Amarth). Five-star albums all, but see the reasons for 1990 and 1995.
2006: A Matter of Life and Death, With Oden on Our Side (Amon Amarth), Christ Illusion (Slayer).
2007: United Abominations (Megadeth), Something Wicked Part 1 (Iced Earth), Gambling With the Devil (Helloween), Fiction (Dark Tranquillity), Land of the Free II (Gamma Ray).
2008: Nostradamus, Something Wicked Part 2, Twilight of the Thundergod (Amon Amarth), Death Magnetic (Metallica). 2006, 2007 and 2008 don't have many albums that really hold a candle to the amazing classics of 1988, but I've bought much more albums frolm these past three years because they're the years I've actively been a metalhead. I think a lot of very solid or excellent albums have come out these years, so it's very much a case of a quantity of good albums.