Tyrantiar - A huge asset to dark teams, this pokemon can act as a defensive rocker, a dragon dance sweeper, or even an assault vest pursuit trapper. Tyranitar is one of the most versatile Pokemon on dark and can fit onto nearly every team, so play around with any options that you think help your team the most!
Note: You can use mega-tyranitar if you want, but the base form is still here as an option.
Crawdaunt - Crawdaunt is a powerful wallbreaker with access to STAB Aadaptibility-boosted Aqua Jet and Liquidation, making it very hard for a lot of Pokemon to switch into. It's dark typing also allows it to get STAB with Knock Off as well, which allows it to have really strong neutral STAB coverage. Overall this Pokemon can be used really well on more offensively oriented teams so that it can poke holes in walls that allow its teammates to sweep.