Major Sets to Look out for:

Hoopa-Unbound @ Darkinium Z  
Ability: Magician  
EVs: 252 HP / 36 Atk / 76 Def / 60 SpD / 84 Spe  
Jolly Nature  
- Hyperspace Fury  
- Gunk Shot  
- Taunt  
- Dark Pulse

Hoopa-Unbound @ Choice Scarf  
Ability: Magician  
EVs: 252 Atk / 104 Def / 152 Spe  
Adamant Nature  
- Hyperspace Fury  
- Gunk Shot  
- Trick  
- Taunt

Hoopa-Unbound @ Tanga Berry  
Ability: Magician  
EVs: 252 SpA / 4 SpD / 252 Spe  
Timid Nature  
IVs: 0 Atk  
- Dark Pulse  
- Taunt  
- Nasty Plot  
- Signal Beam 

What they do:

Darkinium Hoopa-Unbound:
   Lives a Meteor Mash from Mega-Metagross and OHKOs back with z-Dark Pulse
   Outspeeds 4 ev Base 100s and drops them with z-Hyperspace Fury
   Lives a Signal Beat from 252 spa Hoopa-Unbound and OHKOs back with z-Hyperspace Fury
   OHKOs Mega-Gardevoir with Gunk Shot
   OHKOs Mega-Lati@s.

Scarf Hoopa-Unbound:
    Outspeeds and OHKOs Mega-Gardevoir
    Outspeeds and OHKOs non-bulky Metagross-Mega
    Outspeeds and OHKOs Mega-Lati@s
    Outspeeds and OHKOs non-bulky Victini/Mew/Celebi

Tanga Berry Hoopa-Unbound:
     Lures in and beats Colbur Berry Necrozma

What counters these sets (while still being good otherwise):

                      Darkinium Hoopa-Unbound:

Mega-Gardevoir
    Outspeeds and OHKOs with Hyper Beam

Mew
    Mew outspeeds and OHKOs with Leech Life

Victini
    Victini outspeeds and OHKOs with V-Create

Gallade-Mega
    Gallade-Mega outspeeds and OHKOs with X-Scissor

Colbur Berry Necrozma
    Tanks a hit and KOs back with X-Scissor

Buginium Deoxys-Speed
    Outspeeds and KOs with z-Signal Beam
             
      Scarf Hoopa-Unbound:

Mega-Gallade
    Lives a hit and KOs back with X-Scissor

Necrozma
   Lives a hit and KOs back with X-Scissor

Bulky Victini
   Lives a hit and KOs back with V-Create

Bulky Mew
   Lives a hit and KOs back with Leech Life

Metagross-Mega
   Lives a hit and KOs back with Meteor Mash -> Bullet Punch

Buginium Z Deoxys-Speed
   Outspeeds and OHKOs with z-Signal Beam

       Tanga Berry Hoopa-Unbound:

Victini
     Outspeeds and OHKOs with V-create

Gardevoir-Mega
     Outspeeds and OHKOs

Metagross-Mega
     Outspeeds and OHKOs

Buginium Z Necrozma
     Lives a hit and KOs back

Gallade-Mega
     Lives a hit and 2HKOs with X-Scissor

Good Teamates:
    Mega-Metagross
      Deals well with Mega-Gardevoir, some Necrozmas, Mew, Gallade-Mega, and Buginium Deoxys-Speed, all of which threaten Hoopa-Unbound

    Groundium Z Necrozma
      Deals well with both Victini and Metagross-Mega.

Some other, related sets:

Gardevoir-Mega @ Gardevoirite  
Ability: Pixilate  
EVs: 180 HP / 252 Def / 64 SpA / 12 Spe  
Bold Nature  
IVs: 0 Atk  
- Hyper Beam  
- Calm Mind  
- Taunt  
- Laser Focus

Metagross-Mega @ Metagrossite  
Ability: Tough Claws  
EVs: 200 HP / 92 Atk / 100 SpD / 116 Spe  
Adamant Nature  
- Meteor Mash  
- Bullet Punch  
- Laser Focus  
- Bulldoze

Necrozma @ Groundium Z  
Ability: Prism Armor  
EVs: 24 HP / 100 Atk / 252 Def / 132 SpA  
Lax Nature  
- X-Scissor  
- Moonlight  
- Iron Defense  
- Earth Power

Gallade-Mega (M) @ Galladite  
Ability: Inner Focus  
EVs: 252 Atk / 84 Def / 172 Spe  
Jolly Nature  
- X-Scissor  
- Night Slash  
- Bulk Up  
- Shadow Sneak

Victini @ Colbur Berry  
Ability: Victory Star  
EVs: 252 HP / 20 Atk / 100 Def / 40 SpD / 96 Spe  
Jolly Nature  
- V-create  
- Quick Attack  
- Taunt  
- Flame Charge

Mew @ Colbur Berry  
Ability: Synchronize  
EVs: 252 HP / 20 Atk / 100 Def / 40 SpD / 96 Spe  
Jolly Nature  
- Barrier  
- Soft-Boiled  
- U-turn  
- Amnesia

Deoxys-Speed @ Buginium Z  
Ability: Pressure  
EVs: 248 HP / 252 SpA / 8 SpD  
Timid Nature  
IVs: 0 Atk  
- Signal Beam  
- Taunt  
- Amnesia  
- Recover  

Necrozma @ Buginium Z  
Ability: Prism Armor  
EVs: 244 HP / 252 Atk / 12 Spe  
Adamant Nature  
- X-Scissor  
- Swords Dance  
- Calm Mind  
- Rest