That depends what you mean by dark. If you mean which band is the gothiest and dark-obsessed, I'd say probably AFI. If you mean which band plays the heaviest metal, I don't know, but of the ones I've heard, probably Strapping Young Lad. If you mean which band has the harshest, and therefore, darkest lyrics, that's got to be Bad Religion.
"I looked up and saw you;
I know that you saw me.
We froze but for a moment
In empathy."-Rise Against