Some great suggestions here (really happy to see Bolt Thrower mentioned :)). Anyway, here are mine (again, in any order):
CARCASS: HEARTWORK
A full decade before Bullet for my valentine et al tried to combine rock phrasing with detuned heaviness, Carcass released pretty much the last word in this style. Absolutely great album. If you like this, then check out the half dozen or so albums that Arch Enemy (Mike Amott played on Heartwork too) have since released.
AT THE GATES: SLAUGHTER OF THE SOUL
This is how fast songs with detuned guitars should be played :twisted: It is one of the great tragedies of metal that ATG split after this one. The Haunted don't even come close...
ENTOMBED: WOLVERINE BLUES
Absolutely manic effort from a band that were expanding their horizons from their pure death metal roots. Many bands that tried to "move on" from DM lost their power in the process (as did Entombed on subsequent albums). But this album shows how it should be done.
FEAR FACTORY: DEMANUFACTURE
As Kerrang said a the time: a f*cking masterpiece. The tack-sharp, diamond-hard production is perfect for the futuristic tone of the album & Burton C Bell's vocals were at their best here.
MESHUGGAH: CHAOSPHERE
Others have mentioned Obzen which is indeed a great CD, but for sheer unrelenting brutality, Chaosphere trumps it IMHO. Anyone wondering what a 7-string guitar is for should buy this :)
OBITUARY: THE END COMPLETE
The greatest death metal album ever released? I think so. Unlike many/most DM, Obituary don't use blast beats, relying instead on a brutal sludgy mid-paced crawl. It's massively heavy in it's own tortured way & John Tardy is the best DM vocalist by a distance...
