A little while ago there was a thread on the Metal Archives forum about this topic. I got to thinking about it and not a lot of things jumped out at me right away. So, I decided to explore it a little bit more.
Here is my list:
AT THE GATES: "LEGION" (SLAUGHTER LORD COVER)
This cover appeared on the reissued version of the Swedish band's landmark Slaughter of the Soul album. Slaughter Lord is an Australian thrash metal band that never released a full-length album but did release a few demos. They are one of the major influences on the war metal scene.
DIMMU BORGIR: "SATAN MY MASTER" (BATHORY COVER)
Appearing on 2003's Death Cult Armageddon on the bonus disc, this was the first I time I had heard of Bathory. This was an ugly, relentless song. I needed to hear more.
DRAGONLORD: "BLACK FUNERAL" (MERCYFUL FATE COVER)
One of the bands responsible for turning me on to Mercyful Fate was this side project of members of Testament, Sadus, and Nevermore.
IN MEMORY OF CELTIC FROST (TRIBUTE ALBUM TO CELTIC FROST)
One of two tribute albums I picked up before I actually heard anything by the original band. This album featured covers from Morgion, Enslaved, Opeth, Mayhem, Emperor, Grave, Slaughter, and several other bands. This had a dual purpose in introducing me to Celtic Frost as well as a ton of other bands.
METALLICA: "IT'S ELECTRIC", "HELPLESS", "AM I EVIL?", "THE PRINCE" (DIAMOND HEAD COVERS)
Metallica was very clearly influenced by NWOBHM band Diamond Head, as can be seen here. There are only three other tracks on their Lighting to the Nations album that did not appear on this cover album by Metallica.
METALLICA: "MERCYFUL FATE" (MERCYFUL FATE MEDLEY)
The other band that turned me on to Mercyful Fate. This medley was so incredible it did not take me long to seek out Melissa.
METALLICA: "BLITZKRIEG" (BLITZKRIEG COVER)
Another NWOBHM band that was covered by Metallica early on, I used to listen to this on repeat, it was such a great song. The original is even better.
NATIVITY IN BLACK VOL. 1 (TRIBUTE ALBUM TO BLACK SABBATH)
This is the first tribute album I ever bought, but it was a damn good one. Not only did it introduce me to Black Sabbath, but it also helped me get into Corrosion of Conformity, Sepultura, Type O Negative, and featured great tracks by White Zombie and Megadeth.
SEPULTURA: "ORGASMATRON" (MOTORHEAD COVER)
Yes, it's strange. The first time I really remember hearing Motorhead was on this cover by Brazilian thrash metal band Sepultura.
SWALLOW THE SUN: "SOLITUDE" (CANDLEMASS COVER)
An excellent cover of a doom metal classic by this death/doom outfit on their debut. I was pretty quick to get into Candlemass after hearing this.
THORNSPAWN: "DIVUS DE MORTUUS" (NECROVORE COVER)
The 7 inch record this appears on is a bit of an odd item in my collection. Necrovore never released a full-length but they were instrumental on the development of death metal in the 1980's. This originally appears on a four track demo that is very hard to find.
What's next? I have not checked out these original bands yet, but will be soon.
DAWN OF AZAZEL: "PLATEAU OF INVINCIBILITY" (ORDER FROM CHAOS COVER)
DRACONIAN: "FOREVER MY QUEEN" (PENTAGRAM COVER)
IMPIETY: "READY TO EXPLODE" (VULCANO COVER)