A slow Pokemon of course.
The slowest Dark Type Pokemon in OU is Tyranitar which learns Payback via Move Remember.
Source about speed and the payback knowledge came from my head since it learns it at level 41 but doesnt evolve until 55.
There really aren't that many, but I'd say Tyranitar would be your man, as he has nice bulk, great Attack, and is not very fast.
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