I said last week it was getting very difficult to limit this to three. 1985 was particularly difficult, so I increased it to five. I will not go more than that for subsequent years though.
CELTIC FROST: TO MEGA THERION
1985 was a huge year in the development of various styles of metal. At the center of the developing thrash, death, and black metal scene was one band: Celtic Frost. Formed from the ashes of Hellhammer, Celtic Frost utilzed a lot of the same riffing style, heavy and downtuned and fast as hell, and vocals, Warrior's famous grunting. This album is probably closest to thrash metal but the influence that the death and black metal genres took from it are plain as day.
EXODUS: BONDED BY BLOOD
This is pretty much the quintessential thrash metal album. It's fast and aggressive as hell with razor-sharp riffs, great songs, and manic vocals. Exodus is clearly having a blast on this album. It's the band's only full-length under vocalist Paul Baloff who had a very distinct vocal style. Unfortunately, Exodus would never exceed the mastery of their debut. They have put out good albums since this one, but never one quite as good. Bonded by Blood is a thrash metal landmark for very good reason.
HELLOWEEN: WALLS OF JERICHO
As mentioned earlier, 1985 was an important formative year in metal. Power metal also really got going with this release by Helloween. The German band took the sound laid out by groups like Iron Maiden and Judas Priest and played it even more melodically. They still had a lot of the speed of their predecessors, but the bombastic vocal style and melodic sensibilities laid the framework for the European power metal scene.
OVERKILL: FEEL THE FIRE
It's a tough call who came first. There's a lot of historical revisionism that seems to suggest that perhaps Overkill was the first thrash metal band, rather than Metallica. Whether that is true or not really does not matter all that much. This is Overkill's debut album and it is truly one of the great thrash metal debuts, in a genre full of them. The band's punk influences shine through on this, but it is undeniably metal. Overkill is easily the most consistent thrash metal band, never having put out a bad album.
POSSESSED: SEVEN CHURCHES
Another landmark album in the formation of a major metal genre, this time death metal. Possessed is technically a thrash metal band but their heavier, down-tuned riffs, and extremely dark subject matter were influential to groups like Death and Morbid Angel, who took the candle from Possessed to keep the dark light burning. This album has some truly great songs on it and is mostly forgotten by the metal mainstream. But Cannibal Corpse covered one of their songs, so there's that. Criminally underrated by those outside the underground.
Honorable mentions: Anthrax: Spreading the Disease, Artillery: Fear of Tomorrow, Dark Angel: We Have Arrived, Destruction: Infernal Overkill, Kreator: Endless Pain, Megadeth: Killing is My Business...And Business is Good!, Slayer: Hell Awaits, Trouble: The Skull, W.A.S.P.: The Last Command, Watchtower: Energetic Disassembly
Bands that formed in 1985: Candlemass, Carcass, Dream Theater, Exhorder, King Diamond, White Zombie