Perhaps an obvious choice, but the riff at 1:08 (starts a bit later in this youtube clip though) will always be one of the darkest, most unsettling and heavy riffs to me.
Every time I picture buildings just crumbling and the whole of mankind just being slowly dragged into the depths of complete nothingness.
Pure evil and despair, just no hope and it gives me the chills every single time.
I love the entire album, but something about this particular song really hits the spot. The insane rhythms, the intensity, the relentless brutality, it's just everything you'd want in a tech death song.
And goddamn, the section from 4:08 onwards just elevates the song to an even higher level.