I'd have to go with The Final Chapter. From start to finish, this album just seems to be their deepest and darkest album. The guitars have a nice thick quality to them as well. And Peter's vocals here are some of the best he's done.
But I honestly think the "heaviest" album they have is the live one, Hypocrisy Destroys Wacken. I listen to a few songs off it each day and I am constantly blown away by how great it sounds. Peter's vocals are even better on this live album than on any of the studio albums. And the mix is perfect. If this album was in the poll, I would have voted for it. Just listen to the live version of "The Final Chapter." Doesn't get much heavier than that.