It's getting harder and harder to limit these things to just three albums.
DESTRUCTION: SENTENCE OF DEATH
Okay, this was released only in Germany in 1984 and it is only an EP, but it was the first official release by the major German thrash metal band. They classified their sound as "black speed metal", which actually is a fairly good description. It's extremely fast and raw. These are not the most talented musicians at this point, but they are good enough. This was a very important recording in the development of the German thrash metal scene.
MERCYFUL FATE: DON'T BREAK THE OATH
I did Mercyful Fate for my 1983 post, but when an album is this good, why pass it up? I still think Melissa is the better album, but this one is incredible in its own right. It's faster in general and more intense. King Diamond's haunting wail sounds great, although it's definitely a grower. My favorite part is the taunting by King towards the end of "Nightmare". Overall, there are some damn catchy riffs at work here, blazing guitar solos, and of course King's voice. This was one hell of an album. Mercyful Fate never rose quite as high again, but they do continue to put out quality albums, as does King Diamond.
SODOM: IN THE SIGN OF EVIL
This one is also only an EP, but it was massively important in the developing black metal and death metal scenes. The chaotic sound and raw feel of the album was very influential on upcoming bands such as Mayhem and Darkthrone. The EP is often considered one of the main albums in the proto-black metal movement. Sodom would back off from this sound and become a basic thrash metal band on later albums, but this was a huge step toward many of the metal subgenres we currently have.
Honorable Mentions: Anthrax: Fistful of Metal, Armored Saint: March of the Saint, Bathory: Bathory, Celtic Frost: Morbid Tales, Dio: The Last in Line, Dokken: Tooth and Nail, Iron Maiden: Powerslave, Judas Priest: Defenders of the Faith, Manowar: Hail to England, Metal Church: Metal Church, Metallica: Ride the Lightning, Overkill: Overkill, Queensryche: The Warning, Razor: Armed and Dangerous, Trouble: Psalm 9, Voivod: War and Pain, W.A.S.P.: W.A.S.P.
Bands that formed in 1984: Annihilator, Atheist, Blind Guardian, Candlemass, Celtic Frost, Iced Earth, Razor, Sadus, Sepultura, Soundgarden, Stratovarius.