Great answer by Ignis Aqua Herba as always, but I wanted to add something to it.
"Best" is a relative word, anything can be called the best that suits your team, and ngl answering this question would be easier if one knows your team.
As you said you're kind of rekt on by BP Teams, I'll give the ones that are good against them.
Without any further ado, please get ready for a very boring answer.
Yveltal is one of the most viable Dark-types in the metagame. It performs various roles, a Defensive Defogger, a revenge killer, a fast Defogger, a wallbreaker, and whatnot.
If we focus on BP Teams, it has a great matchup against them also, as it threatens Espeon, Scolipede and Drifliblim, plus is immune to Stored Power, and, one of the best things probably, it pulls of a fast Taunt.
Another real nice thing is the act that it's matchup against Espeon is too great, as Espeon tend to come in to Bounce back a Taunt. It can also opt for a mixed set and destroy Mr. Mime's who can setup Calm Mind on it, if it's asleep by Smeargle's Spore.
Outside of it's great MU against BP Teams, it acts as a formidable Revenge Killer with Sucker Punch, threatens Primal Groudon, one of the most common Pokémon in the tier, and can act as a soft-check to Marshadow if ever required.
I'm not stressing on Defensive ones as they don't really have a formidable MU against BP Teams, seeing that it'd lack some firepower that you need.
Plus Offensive ones crit harder bwahaha.
Yveltal @ Life Orb
Ability: Dark Aura
EVs: 4 Atk / 252 SpA / 252 Spe
Rash / Mild / Naive Nature
- Dark Pulse
- Oblivion Wing
- Sucker Punch / Knock Off
Ay so, pretty normal set. The last move is choice. Knock Off removes some items but fails to do anything else as Dark Pulse covers almost everything. Sucker Punch is kinda better as it lets you revenge kill like loads of foes so ggswp. Nature is also choice Rash / Mild is preferred but if you want some speed go with Naive.
Ah yes. I love this thing. It can Taunt anything, trap and Perish crittes out, or just trap and eliminate other critters that aren't letting your other teammates sweep, and whatnot.
Against BP Teams, it can Taunt and Perish Song them, which'd mean that they would have to break their chain as start again if they don't want to be banished to Willy Wonka's Chocolate Factory (aka faint).
It can also set up a Substitute which'd make it immune to Smeargle's Spore, and, seeing that BP Teams don't carry any firepower outside Espeon, you'd be left with a Sub likely till the end of the match and wreck havoc on the opposing team. It can Sludge Wave it's way past Mr. Mime's, Shadow Ball Drifblim and Espeon, and uh Taunt the rest.
It's only problem is it's weakness to Stored Power, which'd mean, unless you run Sub, this may lose to the BP Chain. Play with it wisely, and the match is yours.
Another nice thing to note is it's great MU against Espeon, the Magic Bouncer. Of course no one would try to send an Espeon on a manual switch to the Gengar but yo what's the harm mentioning.
Gengar-Mega @ Gengarite
Ability: Shadow Tag
EVs: 252 SpA / 4 SpD/ 252 Spe
- Shadow Ball
- Sludge Wave
Aaaa what's there to explain lol. Sub turn one and keep Ball-ing, Sludge-ing and Taunt-ing things.
I don't really have much experience with Deo-S, and so I won't be commenting much on it.
Deoxys-Speed is what HO Players want from their hazard setter: xxtremely fast, great MU against Magic Bounce users, can stop the opponent from setting their hazards, and can set multiple of it's own hazards.
But sadly, it's usage is limited to HO Team archetypes only, as BO and Balance don't like to waste a whole teamslot for something that can't do what they want it to do. But, if you use HO, it would rock in your team, and you may have already put it on your team also.
Against BP Teams, it doesn't really have a bad matchup. It can Taunt them easily and can set its hazards to deter the few Sashes that they run. It also has a nice MU against Smeargle, whose Spore it can bounce back with Magic Coat. Watch out for Nuzzle though, that may deter what your poor Deoxys can do.
Another great thing is that it has a very very great MU against Espeon, who can Bounce back your hazards and your precious Taunt: Skill Swap. It can swap Magic Bounce with the mediocre Pressure, and boom, you get a free chance to set your hazards or Taunt something.
All in all, Deoxys-S would only be useful if you run HO, otherwise it ain't so good.
Deoxys-Speed @ Focus Sash
EVs: 252 HP / 4 Def / 252 Spe
- Stealth Rock
- Magic Coat
- Skill Swap
Hehe I explained the whole set beforehand lolers.
Another nice strat
Zygarde-50% @ Safety Goggles
Ability: Power Construct
EVs: 252 HP / 4 Atk / 252 Def
- Thousand Arrows
- Glare / Toxic
Ok so, this set is kinda pog especially against BP Teams. Haze makes all the art changes the poor opponent acquired go poof, which is so nice. Use this set with some mind actually as Mr. Mime can Taunt you, and setup their own Calm Mind's. Safety Goggles makes it immune to Smeargle's Spore, and opponent's reaction is so busted when they get to know that they got scammed. I've never laughed so hard won't lie.
Glare or Toxic is choice, Toxic punishes SupportCeus formes that don't like to be on a timer, whereas Glare spreads Paralysis, which is better against the likes of Marshadow and all.
Thousand Arrows is the only good move to fit this set otherwise this thing complete Taunt bait :(
Rest is pretty much very reliable, seeing that Zygarde's bulk is so gargantuan, you'll be able to pull of more than a few ones.
Source: Source? Huh I got no source gotta trust me hehe
Bwahaha that's the end of this boring answer lol hope it helped, feel free to Wall me or Discord if you need any other help in AG or any OM :D