Mewtwo
Pros:
- Great SpA and Spe at 154/130 base respectively.
- Amazing special movepool, it can run BoltBeam, as well as having strong STAB (Psystrike) and coverage for weakness (Aura Sphere). There are just so many options.
- Doesn't have bad defenses either, at 90 base each.
Cons:
- Common weaknesses. Many Pokémon run Knock Off and Shadow Ball.
- Although 90 base defenses aren't necessarily bad, it's more of a glass cannon.
Mewtwo @ Life Orb
Ability: Pressure
EVs: 4 HP / 252 SpA / 252 Spe
Timid Nature
- Recover
- Psystrike
- Ice Beam
- Thunderbolt / Aura Sphere / Fire Blast / Shadow Ball / Energy Ball
Eternatus
Pros:
- Again, good SpA, Spe, and HP.
- Another decent movepool with options like Dynamax Cannon, Sludge Bomb, Flash Cannon, Flamethrower, and Shadow Ball.
- Access to utility moves like Recover, Cosmic Power, and Toxic Spikes.
Cons:
- Weird typing. It has 7 resistances, but is weak to common coverage moves like Psychic, Ice Beam, Dragon Pulse, and Earthquake.
- Doesn't have an amazing level up/TM/TR learnset, with only the mentioned moves above.
Defensive Set:
Eternatus @ Black Sludge
Ability: Pressure
EVs: 252 HP / 128 Def / 128 SpD
Bold Nature
- Recover
- Cosmic Power
- Toxic
- Mystical Fire / Venoshock
Offensive Set:
Eternatus @ Life Orb
Nature: Modest
Ability: Pressure
EVs: 4 HP / 252 SpA / 252 Spe
- Dynamax Cannon
- Flamethrower
- Sludge Bomb
- Recover / Flash Cannon / Shadow Ball
Zeraora
Pros:
- Only physical attacker of the three options.
- Great physical movepool.
- Fantastic Speed and decent Attack.
Cons:
- Weak to coverage moves like Earthquake.
- Has a lower BST than either Mewtwo or Eternatus, with only 600.
- Low defenses.
Zeraora @ Choice Band / Life Orb
Ability: Volt Absorb
EVs: 252 Atk / 4 SpD / 252 Spe
Adamant Nature
- Plasma Fists
- Close Combat
- Play Rough
- Outrage / Blaze Kick / Hone Claws
Really, just pick whatever your team needs. I personally would go with Mewtwo, because of its large movepool and great stats. Just make sure your team is balanced. Also, you're going to have to EV train if you want to get anywhere in this tournament.