"Show no Mercy" and "Hell Awaits" are fucking awesome albums. First Slayer song I heard was Black Magic off "Live Undead" I was hooked after the into riff.
Yeah there's better thrash bands (Kreator, Old Sepultura, Dark Angel, Overkill, Sodom etc) but Slayer were one of the first and dare I say the most influential.
If Dark Angel could have afforded decent production, "Darkness Descends" would have killed Reign In Blood it's easily 10* better. Listen to some of those riff's, half of them are burried in the production but damn, greatest thrash album ever no question..