Well, it's a toss up between thrash and death metal. Maybe I'll do death too, but I'll go with thrash here because two people already have, and both are far different than mine.
Six thrash albums I deem essential (sorry, tried but couldn't eliminate one of these, so there are six). In no particular order:
Kreator - Extreme Agression
My favorite album from my favorite band of all time (and that's a bold statement for me).
Sabbat - Dreamweaver
I will put it very simply. If you do not have this album, you have problems way beyond my help.
Dark Angel - Time Does Not Heal
Unbelievable riff-fest that amazes me every time I listen to it. The best thing Hoglan will ever touch by a mile.
Vio-lence - Eternal Nightmare
Sometimes there is no way to explain why you love an album so much, you just do. Count this as one of them.
Hypnosia - Extreme Hatred
Best thrash album to come out in the last ten years. A little Kreator worship never hurt anyone...
Testament - First Strike Still Deadly
At first I couldn't decide between Legacy and New Order, but the more I think about it, this is the right choice. These songs are billiant in their original versions, and somehow, leave it to Testament, these ones are better.