Here's a list that tries to describe extreme and more experimental forms of metal without being only about the figureheads that every fan already knows about:
Adramelech - (1996) Psychostasia
Flawless and typical finndeath. Should please anyone with DM experience.
Alchemist - (1993) - Jar of Kingdom
First album from the psychedelic death band from Australia. Very raw and their most inaccessible release, but worth the effort.
Anacrusis - (1991) Manic Impressions
Technical and melodic thrash, an interesting mix. All this band's albums are worth a listen, by the way.
Anathema - (1995) Pentecost III
Not an unknown band, but this EP is very much their best work.
Atrocity - (1990) Hallucinations
Now this is a landmark. One of the first if not the first technical death album and a concept album at that. Gorguts actually cited this album as inspiration for Obscura and The Erosion of Sanity.
Behereit - (1993) Drawing Down the Moon (no full album youtube link found)
Very doomy black metal with an intense occult ambiance. Generally loved or hated.
Belkètre - (1996) Ambre Zuèrkl Vuorhdrévarvtre (no link, 2deep4u)
One of the rawest black metal album from what is, in my opinion, the second best band in Les Légions Noires.
Berserkr - (1994) The Voice of Our Ancestors (no link, probably too politically incorrect for youtube)
This album is a very diverse mix of death, thrash, sludge with a healty dose of "southern" US feel. I guess I should warn some people: their political stance might not be for you.
Bethlehem - (1994) Dark Metal
Another doom/black album, with one very "unique" singer. Very modern and clean production, unusual in the genre.
Cenotaph - (1992) The Gloomy Reflection of our Hidden Sorrows
Death metal from Mexico with a very bad production and an extremely heavy sound. The subsequent albums have a way cleaner production and are also worth a try.
Confessor - (1991) Condemned (no link found)
One legendary technical doom album. Probably the basis for Meshuggah's current drumming and in my opinion the best drum performance I've ever heard.
Convulse - (1994) Reflections
https://www.youtube.com/watch?v=StM6AZqA2qA (only one track)
This band is a classic swedish death band whose first release, World Without God, is quite famous. They went toward a death 'n' roll sound with this one while keeping their melodic side; a very underrated LP.
Demilich - (1993) Nespithe
https://www.anentity.com/demilich/download.php
A cult milestone in a metal fan's descent toward the most inaccessible but rewarding stuff. The strange structures and harmonies coupled with one of the most inhuman voices are still unmatched to this day. A nice bonus is that the entire band discography is freely available on their website.
diSEMBOWELMENT - (1993) Transcendence into the Peripheral
Another one album band disappearing after leaving us with something incredible. If I try to qualify it, it'd be psychedelic death/doom, and an extremely heavy/oppressive one at that.
Disharmonic Orchestra - (1990) Expositionsprophylaxe
Not an unknown band, but their first LP is too cult to pass. A technical deathgrind gem that nobody should ignore without lending it an ear.
Enslaved - (1994) Vikingligr Veldi
https://www.youtube.com/watch?v=coJ_O3YYcNY
First album from a very famous band now. A melodic and quite folk (but without the gentle folk instruments) black metal release. Their finest work for a lot of fans.
Entombed - (1993) Wolverine Blues
https://www.youtube.com/watch?v=5GkoShU3xX4
A very divisive release from one of the top swedish death band. For a lot, this first death 'n' roll album is the beginning of the end, but for others, this is their swansong, where they really realized their full potential.
Epitaph - (1992) Seeming Salvation
https://www.youtube.com/watch?v=qLqFS_UKpxQ
A VERY underrated melodice thrash/death release from a swedish band that didn't sound exactly like their Gothenburg peers.
Exhorder - (1992) The Law
The very first groove metal band, an inspiration for Pantera but a bit more hardcore than their pupil (well, more more like classmate, really).
Fear Factory - (1992) Soul of a New Machine
https://www.youtube.com/watch?v=sBiYm6kgdL4 (no full album link, sadly)
The only non mainstream Fear Factory album (not counting the Fear is the Mindkiller EP), a deathgrind affair with the industrial touch that'll become their trademark.
Gehenna - (1994) First Spell
https://www.youtube.com/watch?v=VoWojybrN1I
A unique black metal band, with a very melodic, synth backed sound, not unlike Emperor's first, but with much less speed and blast beat. A very good and soft entry to the genre, I'd say.
Godflesh - (1989) Streetcleaner
https://www.youtube.com/watch?v=HfN7OR5q_xw
Nobody can describe extreme metal without mentioning Godflesh's first full release, fusing industrial, noise and death. One of the most vicious album to me, making Swans sound like a nursery rhyme.
Godkiller - (1996) The Rebirth of the Middle Ages
https://www.youtube.com/watch?v=QC5LhuLyanA
Quite cult, again. Especially for being from Monaco, not the most metal filled country. This album is the dictionary entry for melodic black metal, being even better than heavyweights like Dissection or Taake; at least on Windir's level. Caution: the voice is not easy to take seriously.
Golden Dawn - (2003) Masquerade
https://www.youtube.com/watch?v=Z_jXsHaOalo
Neoclassical melodic gothic metal, enough said. One of the most diverse release and band in general. Highly recommended for those a bit bored by their current listening material.
Gorguts - (1998) Obscura
https://www.youtube.com/watch?v=X1X0Bejwnko
Not an unknown name at all, this album is a controversial one. On one hand, it's still one of the most complex, exhausting and unreachable release, but some (philistines) say that this is just pointless ear crushing noise with a hint of death metal and fusion flavour. Your call, but this is DEFINITELY not for someone who never heard death metal.
Graveland - (1994) The Celtic Winter
https://www.youtube.com/watch?v=q3eVYBpDQ3U
What was supposed to be Graveland's first album, with a sound very similar to the following Carpatian Wolves LP. That sound will only last two albums before mellowing into an easier and more "casual" form but marked the minds by the theme they evoke: the Dark Ages, and not the castles and princesses part, but more like vicious barbarian raids, pointless battles in the cold mud and torture chambers.
Helheim - (1996) Fenris
https://www.youtube.com/watch?v=YAuCNDw5tUs
Part of the very select club of industrial black metal with Thorns and Mysticum, it's the lesser known one of the three.
Hirilorn - (1998) Legends of Evil and Eternal Death
https://www.youtube.com/watch?v=GE6hPQ20m9g
In my opinion the very best melodic black metal album there is. Simply no weakness, and a very lead guitar driven sound that should appeal to a lot.
Horn - (2008) Naturkraft
https://www.youtube.com/watch?v=XeDDBPF-jkI
Another melodic black metal band with a very clean (for black metal) and modern take on the genre. Very good entry point to black metal.
Ice Ages - (1997) Strike the Ground
https://www.youtube.com/watch?v=ThZrG8n33YM
This is actually not metal, but something qualified as "EBM/Electro/Industrial" on metalarchives. Not listening to electronic music at all, I can't say if this is accurate or not, but this album really clicked with me. This is a one man band by one of the guys behind the very famous Summoning (this is better, in my opinion).
Immortal - (1995) Battles in the North
https://www.youtube.com/watch?v=bt1DzuiuW88
I hesitated about this one, because anybody into extreme metal knows Immortal, but I thought that this release was a bit too eclipsed by Pure Holocaust, which is not as good. This release has a very similar style to Pure Holocaust, but with a production so cold it's dry, which gave its name to the unofficial genre "blizzard metal".
In Flames - (1995) Subterranean
https://www.youtube.com/watch?v=suzldZRB3OU
Everybody knows this very cheesy band, but not everybody knows that they did an incredible LP (Lunar Strain) followed by an even better EP (this one) at their beginning. A must have for fans of the Gothenburg sound.
Infester - (1994) To the Depths… In Degradation
https://www.youtube.com/watch?v=emSDcQ89Ne8
Cult release considered amongst the darkest and most unsettling death metal albums. Not for the inexperienced.
That's it for now, I went through half of my favourite albums, will do the other half if there's some demand.