I feel like a princess in this thread, because I don't like a lot of the heavier things out there, and I'm at the point where a lot of the heaviest stuff is just comical to me (I sing "The Entrance of the Gladiators" - you know it as "the circus theme" - in my head every time I hear Slayer; they're a joke to me) but for me, "heavy" in terms of sound, Orgasmatron by Motorhead, Dehumanizer by Sabbath, the second G//Z/R record.
"Heavy" as in subject matter? No question, hands down, "Dirt" by ALice In Chains. That's a BRUTAL record, IMO.