As for what chaos army looks the most grimdark, it would depend on the exact scheme but I would say it is a tie between black legion with lots of spikes and skulls, and a more subdued paint scheme for night lords with lots of flayed skin and dismembered body parts.
That said, story-wise I think Word Bearers make the best match. Sure, your grey knights could well be hunting down psychopathic murderers, but for the purpose of a compelling narrative its a better opposing force for your grey knights if those murders are not the goal but serve some greater and darker cause.
After all, if you are painting (figuratively) your grey knights as the shining (again figuratively) defenders of humanity who are the only ones who stand between the forces of chaos and the utter damnation of humanity, it is a better chaos foil to have the Word Bearers, a legion whose very purpose is the damnation of humanity, who freely implement demons the very force that the grey knights were formed to vanquish.
So, sure, you could have your grey knights trying to stop the night lords from killing a bunch of people because they are evil and bad and want to scare people... but really, doesn't it make a better story for your grey knights trying to stop the word bearers from killing a bunch of people because they are evil and bad and the murders will open a daemonic gate that will flood the planet with daemons, damning everyone left on the planet and weakening the veil between real space and the warp for the entire subsector?
Now, that's not to say that the night lords might not be involved in such a plot, but it is a more natural fit for the word bearers as it is kind of their thing.
Incidentally, if you want to see more excellent word bearers, I would recommend this thread (disclamer: not my work) http://www.bolterand...rd-bearers-log/
Edited by Dr_Ruminahui, 20 October 2020 - 11:48 PM.