Oh... so many...
But if I had to pick one, I'm thinking I would go with Carcass - Symphonies of Sickness. The first two Entombed albums are right there too though.
In a few years, I may very well be saying Nihility, but as much as I think that album is brilliant, I always need some time before truly putting it in the category of 'best ever'. It's on it's way though...