Not sure if my participation in this has scared everyone off or made it a suddenly less cool thread, but anyway...
133. "Black Sabbath" --- Black Sabbath --- 1970
Need I say more? If you want to hear the first demonic mewling screams from the baby that would grow up to be heavy metal, this is the only place to start. When those heavy guitar chords ring out and the rain lashes down against a backdrop of mournful bells, you know you're in the operating theatre watching in horrified fascination as something truly beautiful and terrible is born, and the world will never be the same.
And don't tell me "Paranoid" is better, even after it won the smackdown in my journal!