Bisharp was a no-brainer for my first choice. Although it's not as good in SM as it was in ORAS, it's my favorite dark type Pokemon, and provides as a solid utility and wallbreaking teammate. It's set is standard. Swords Dance is for boosting it's attack even higher than before, Knock Off is it's best STAB dark move (and is also a great utility move for removing items) Sucker Punch is for priority, and Iron Head is for secondary STAB coverage. It's running 252 speed and Jolly to ensure it's as fast as possible for outspeeding many other threats. It's running 56 Special Defense to ensure it'll survive a Moonblast from Diancie-Mega (without hazard damage), allowing it to retaliate with an OHKO from Iron Head; And, of course, the remaining 200 EVs are dumped into attack to improve it's general power.
Mandibuzz was one of my favorite Dark type Pokemon in ORAS as well, and it still is in SM. It's an amazing tank, sporting great HP and solid Defense and SpDefense. I originally added it in to act as a utilitarian Pokemon for removing hazards for Bisharp, but after some testing, I learned that Mandibuzz can tank just about
any incoming hit. I also found a fascinating damage calculation while laddering:
252+ Atk Garchomp-Mega Dragon Claw vs. 252 HP / 76+ Def Mandibuzz: 144-169 (33.9 - 39.8%) -- guaranteed 3HKO
This set can take up to 3 Dragon Claws from a 252+ Atk Garchomp-Mega, who sports a monstrous 170 Attack. And Mandibuzz can switch into it and roost it away with ease. Even Outrage only does 50-59%! Obviously, it's ideal to still not switch into an attack like this unless necessary, but it's quite settling to know that Mandibuzz can very well potentially do it when needed.
So, in the end, Mandibuzz uses Roost, Defog, Toxic and Knock Off to act as a multi-purpose tank and utility Pokemon, assisting the team in all sorts of different ways.
Sableye-Mega is one of the best, if not the best, defensive mega out there right now. It has great Defense and Special Defense, and has a whole load of different moves to assist it in getting the job done. Without any modifiers, it serves as a great physical wall, sporting high Defense and Will-O-Wisp, which slices the target's Attack in half. Special Attackers coming in? Don't worry about it! Calm Mind boosts SpA and SpD, allowing it to take more hits and deal more damage at the same time. Snarl is the preferred attacking move. Even though it only has 55 BP, it lowers the target's SpA, making it even easier than before for Sableye to soak up hits and then Recover them away with no issue. Although, if you think that's not necessary and you're worried more about power, you can run Dark Pulse over Snarl. I've had countless occasions where people have forfeited because they couldn't figure out a way to take down the almighty Sableye-Mega after a couple of Calm Minds.
Muk-Alola is a staple to this team for several reasons. It's fantastic Defensive typing of Poison/Dark grants it only one weakness; Ground. For this reason, it takes Fighting and Fairy attacks neutrally, allowing the team to stand out more in the defensive sector of the game. With Assault Vest, it's SpDefense raises to 354, allowing it to take several special attacks and return the favor with a STAB Knock Off or Gunk Shot. Although Stone Edge is on the typical set, this team runs Tyranitar, which delivers much more pain in the Rock-type section, so Stone Edge isn't necessary. I run Shadow Sneak instead. Although it's not going to do a lot of damage to anything, it deals a fair amount, allowing it to kill off faster threats that are close to being KOd. Pursuit is also ran so it can switch into a better matchup, and catch the victim with some STAB damage on the way out if it switches.
Tyranitar is also a very important part of this team. Thanks to Sand Stream, Tyranitar is transformed into a monstrous special wall, with 429 SpAttack that can absorb an attack with ease and return with a powerful STAB attack. Stone Edge and Crunch are Tyranitar's best STAB moves, and Pursuit is run for the same reason that Muk runs it. Earthquake is used simply as coverage for Steel types that Stone Edge won't be doing much too. Although it is considered a wall, it's important to preserve Tyranitar as long as possible since it doesn't have any form of recovery in this set. Of course, if you'd like, you can switch out Choice Band for Leftovers for the small bit of passive recovery, but it's recommended that Choice Band is kept for more brute power.
And finally, the speed MVP of the team, Greninja. It's Specs set allows it to quickly take out opposing threats that are weak to one of Greninja's attacks, at a jiffy speed of 377. Although, since Greninja dies very easily, it's recommended that the opposing team is softened up first, and then Greninja is used as a late-game cleaner. Extrasensory and Ice Beam are used as coverage against the likes of Landorus and other opposing Dragon and Fighting types. Hydro Pump and Dark Pulse are simply used as more powerful coverage, Dark Pulse being mainly coverage and Hydro Pump being Greninja's most potent and sweep-potential move. Late-game cleaning with Specs Greninja can be tough, as if one of the remaining opponents resists the move your locked into, you won't be able to break past it. It's recommended that you clear out any water-resistant Pokemon beforehand, allowing Greninja to either seriously damage or completely clean up the rest of their team with Hydro Pump.