Intriguing.
I agree with you that aesthetically the Chaos Dwarfs are more grimdark. At the end of the day, no matter how evil you make an elf, it's still only an elf!!

Also the beards, armour, fire, magma and daemonic influence gives them the edge.
If we consider the lore angle, I think a case can be made for the Dark Elves. The Chaos Dwarfs view other races as beneath them and expendable. As such, they take prisoners/slaves to perform the worst of the labor in pursuit of building their empire/civilization. While they are undoubtedly cruel, I don't think they go as far as the Dark Elves who take great pleasure in torturing their victims for the sheer enjoyment of it. The Dark Elves also have all their strange blood rituals. Lastly, the Chaos Dwarfs are more honorable (among their own kind!) than the treacherous Dark Elves.
As for the other evil races, Orcs/Goblins and Skaven are too silly. Beastmen are too stupid. Daemon models look a bit too cartoony. That leaves us with the WoC and Vampire Counts as the only other viable contenders (although I feel that the Chaos Dwarfs and Dark Elves have them beat).