My list:
The giants of early metal were Black Sabbath, Led Zeppelin and Deep Purple in that order. For Sabbath, the first three albums are definitely essential, and Volume 4 is pretty close. For Zeppelin, the first five albums are the absolute essentials. For Deep Purple, Machine Head is probably the only must-buy. Steppenwolf deserves a mention, even though they’re more proto-metal than anything else. They’ve got a good greatest hits package that has all the necessary songs.
After that, you get into the new wave of British heavy metal and early power metal with bands like Judas Priest, Iron Maiden, Diamond Head, Saxon, Motorhead and Manowar. Others have covered Maiden, Priest and Motorhead, so let me just toss out a couple albums for the rest: Diamond Head’s essential recording is Lightning to the Nations; Saxon’s is probably Strong Arm of the Law; and Manowar’s is Hail to England.
For hair metal, you gotta have Van Halen’s 1984, Def Leppard’s Pyromania, and something by Motley Crue (I’m skimping on the umlauts cause otherwise this would be a pain in the ass to type). You might find Guns ‘n Roses’ Appetite for Destruction in a list like this, but they were barely hair metal.
The kings of thrash were of course Megadeth, Metallica, Anthrax and Slayer. Slayer’s only essential album (IMO) is Reign in Blood, and Anthrax’s is Among the Living. For Metallica, the first four albums are all essential. Megadeth’s most solid early albums are Peace Sells and So Far, So Good…So What?. If you like thrash, you should probably also check out Kreator, Sodom, Forbidden, Flotsam and Jetsam, Exodus and Testament. Flotsam and Jetsam’s first album, Doomsday for the Deceiver, is notable because Jason Newsted was the bassist, and it’s easy to see why Metallica decided to pick him up.
Around the same time as the early thrash, you also had the first memorable doom metal bands. Sabbath worship had been widespread on since 1970, but the only band from before 1985 or so who’s remembered for it is Pentagram. Once you got acts like Candlemass (Epicus Doomicus Metallicus, greatest hits), Trouble (Psalm 9) and Saint Vitus (Born Too Late), it really took hold and has been strong ever since.
Venom deserves note for being the first really Satanic metal band. They’ve got a two-disc set, In League with Satan, that’s pretty much all a casual fan could want.
That’s the first fifteen or so years in a nutshell. I’ll post later stuff later.