Alright, time for me to tell y'all some of my Dark Monotype experiences.
Malamar for A/S Rank:
Ability: Contrary is the best, but Infiltrator could also be used.
Moves: Psycho Cut, Night Slash, Superpower (goes well with Contrary), Hypnosis/Pluck (for those berries)/Light Screen/Destiny Bond
Malamar has the coveted Ability Contrary, which makes it an absolute beast if it can set up while still doing some very nice damage with Superpower. It also has Psycho Cut and Night Slash as its two best STABs as of right now. Those three moves just simply must be on your Malamar. Your fourth move slot if your choice, however. Hypnosis, while having low accuracy, could be useful for Pokemon that are particularly annoying. Pluck gives some decent coverage while also stealing any opponent's berry, which could be useful for the early game, when Sitrus Berries are still being used. Light Screen is just there to boost your team's Special Defense, and Destiny Bond is very nice, since Malamar doesn't have the best Defenses. Overall, this Pokemon has quite a bit of utility if it manages to set up.
Greninja for S Rank:
Ability: Protean
Moves: Surf/Hydro Pump/Waterfall, Dark Pulse/Night Slash, Extrasensory, Spikes/Toxic Spikes/Haze/Mat Block/Acrobatics
Greninja is probably one of the best Dark types ever. With the great insta-STAB ability, Protean, this Pokemon can be a menace as a Physical, Special, or Mixed Attacker! The first three moves are pretty obvious things you'd want to have one a Greninja. As for the last one, Spikes and Toxic Spikes are great for setting things up, but you probably want to kill things before they kill you. Haze is only for the situation where Pokemon can set up. Mat Block is more used for Doubles, since most people know what big trainers' teams are and don't need to scout, and Acrobatics can just be used for coverage. Greninja is definitely a great glass cannon for a Dark Monotype!
Liepard for C/B Rank:
Abiltiy: Prankster
Moves: Fake Out, Night Slash/Sucker Punch, Play Rough, Sand Attack/Taunt/Attract/Charm/Yawn/Encore
Liepard is a very fast Pokemon with decent offenses. Plus, most of its moves gain some form of Priority. Fake Out is a first-turn attack, Night Slash or Sucker Punch is Liepard's best STAB, Play Rough is for coverage and that slight chance of lowering Attack, and those last few moves all are moves that have Priority from Prankster, each being useful in some way. Attract and Yawn are for stopping the opponent, Taunt and Encore are for Pokemon setting up, Charm is simply for lowering Attack, and Sand Attack, my personal favorite, can be used to try to make the opponent miss every attack. Liepard was especially useful for those battles meant to be lost or just hard battles in general, seeing that Sand Attack has priority and can be used so that every boss could potentially miss. In addition, Liepard is one of the earliest Pokemon you can get, so it's great for the beginning-middle of the game!