Dark is currently one of the best offensive types, if not the best.
If we are looking at it at a ratio of super-effective/not-very-effective&immunity, Bug gets the first spot.
However, we are also looking at the pokemon we can work with.
To finish with dark, it can also be used defensively. Remember the dark/ghost combo, which has 1 weakness to the seldom fairy offense.
Moving on with the topic, once more, Bug.
Those little fellows, apart from their mediocre stats, not being able to excell anywhere, are nerfed once again this gen, with an extra resistance, fairy.
Now 7/18 pure thingies cannot be touched. This translates to 1/3rd of all pokemon in existence to be resistant to it and almost 1/3rd of the OU tier, while it's super effective against only 9 OU pokemon (including BL).
Defensively, they can't really do anything. Having a resistance to ground is nice, but birdspam, fire-attacks and the ever mandatory rock attacks (AND SR WEAKNESS) don't give a good prognosis for the little people.