Obviously Slayer. The general consensus seems to be that Reign In Blood is their all-time best album. (also early albums > newer albums)
Also, depending on what genre you like...
Death metal: Death, Nile, Behemoth (new); Black metal: Darkthrone, Deathspell Omega; Doom metal: Sunn O))), Candlemass;
Power metal: Symphony X, Firewind; Thrash metal: Exodus, Dew-scented, Malevolent Creation;
Metalcore: Heaven Shall Burn, Darkest Hour, All That Remains; Grindcore: Napalm Death, Pig Destroyer
There are, of course, many more staples to each subgenre, and also more obscure/defined subgenres, but this will give you a general idea of the better material you will find.