Krookodile is a good dark type to have, you get Sandile at Desert resort, so you get it pretty early in the game. It has good attack and decent HP and Speed.
If you happen to get a Zorua or a Zoroark, it makes a good dark type. It has a good Attack, Special Attack, and Speed.
For this Pokemon, you'll have to wait. You get Deino at Victory Road, but Hydreigon is a good Pokemon to use. It has good Attack and Special Attack.