>>56050587Kingambit isn't the most used OU Pokemon, Great Tusk is. Kingambit is #2. Plus, popularity =/= power. Dark being a mediocre type is actually one of the reasons why Darkrai was even allowed in OU.
And just because you need a Dark type on every team because otherwise you get dogged by Skeledirge, Dragapult, and Gholdengo doesn't mean Dark is good, it means that Ghost is overpowered to the point where you need to use a lesser type to keep it in check.