Deviant Art by supersleuth10

Metagross

Type:
Base Stats: 80 HP / 135 Attack / 130 Defense /  95 Special Attack / 90 Special Defense / 70 Speed
Weaknesses:    x2   x2 x2    x2
Resistances:  x0.25   x0.5  x0.5   x0.5  x0.5  x0.5   x0.5  x0.5  x0.5
Immunities:  
Abilities:  Clear Body - Prevents Stat Changes except from the User
Hidden Ability: Light Metal - Halves the User's Weight 
 

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Metagross @ Psychium Z  
Ability: Clear Body  
Level: 50  
EVs: 252 HP / 116 Atk / 36 Def / 12 SpD / 92 Spe  
Adamant Nature  
- Meteor Mash  
- Zen Headbutt  
- Earthquake / Hammer Arm
- Protect

Milestones:
- Z-Move Shattered Psyche under Psychic Terrain can OHKO many common threats if used sucessfully.

Here we have the Psychium Z Metagross set with a standard three attacks options. In retrospects, Metagross is a rather simple Pokemon to use in terms of movepool, though most of the attacks listed do have specific intentions. STAB Meteor Mash allows Metagross to hit opposing Fairy-, Rock-, and Ice-type Pokemon such as Alolan Ninetales, Gigalith, Tapu Lele, etc for heavy damage with the chance to rais the attack stat by one stage. Zen Headbutt is another STAB option that's now potentially "stronger" in due part of the presense of Tapu Lele's ability to summon Psychic Terrain. Under the field condition, Zen Headbutt hits a 120 Base Power which can do some a considerable damage against opposing Pokemon, though not enough for the OHKO. The last two slots are between Earthquake or Hammer Arm which both have their merits. Earthquake allows Metagross to his opposing Electric-, Fire-, and Steel-types for super effective damage meanwhile Hammer Arm hits opposing Normal-types like Snorlax and Porygon2 hard. 

One of Metagross's best selling points is to use the Psychium Z item in conjunction with Clear Body and the Psychic Terrain. Under the field condition, Z-Zen Headbutt, or Shattered Psyche reaches a base power of 240 before STAB, making this one of the strongest Z-attacks in the format. Given how Metagross has a base attack of 130 and an ability which prevent stat drops, a Shattered Psyche from this Pokemon will hurt even the bulkiest of walls. In order to achieve this feat, make sure to pair Metagross with Tapu Lele to summon the Psychic Terrain. Even so, Tapu Lele is a great partner to pair with Metagross as it covers it Dark type weakness while form part of a Steel-Dragon-Fairy core. The speed investment listed allows Metagross to outspeed neutral base 80s, though more can be added if chosen.

Assault Vest 

Metagross @ Assault Vest  
Ability: Clear Body  
Level: 50  
EVs: 252 HP / 116 Atk / 36 Def / 92 SpD / 12 Spe  
Adamant Nature  
- Meteor Mash  
- Zen Headbutt  
- Earthquake  
- Bullet Punch

Milestones:
-Same Offensive Achievements
-AV to take Special Attacks Better

This Metagross set is built towards takes more specially-based hits while dish out damage against the opposing side. One of the most common and threatening special attackers in the VGC 17 format is Tapu Lele thanks to its Psychic Terrain-boosted STABs which can 2HKO, or potentially OHKO any target. With the Assault Vest and the double resistance to Psychic STAB, Assault Vest Metagross makes for one of the best Tapu Lele responses. In addition, Metagross can survive a number of specially-based Z move attacks such as the Hydro Vortex in the Rain from Duck, an Eruption from Drought Torkoal, 2 Thunderbolts from Tapu Koko in Electric Terrain, and a host of other attacks. In terms of movepool, most of Metagross's attacks are largely unchange from the first set, except Bullet Punch is opted for priority. For a list of damage calculations with the Assault Vest, refer down below:

Metagross @ Choice Band  
Ability: Clear Body  
Level: 50  
EVs: 220 HP / 252 Atk / 4 Def / 4 SpD / 28 Spe  
Adamant Nature  
- Meteor Mash  
- Zen Headbutt  
- Earthquake  
- Bullet Punch / Hammer Arm

Milestones:

-Guaranteed OHKO against "frail" Tapu Koko, Alolan Raichu, and Xurkitree with CB Earthquake

- With Psychic Terrain, has a 15/16th chance to OHKO Garchomp with Zen Headbutt
-Can 2HKO most Pokemon with approriate attack.

With the base 135 attack stat, Metagross makes for a great Choice Band user who can essentially hit many targets its STAB and coverage options listed down. Choice Band Meteor Mash has essentially OHKO most Fairy-types in the format bar Tapu Fini, not to mention has a 20% chance to raise Metagross Attack further. As mentioned before, if Psychic Terrain is up, Zen Headbutt will deal further damage, and pairing it with the Choice Band can net some rather crazy OHKOs. Some examples of these offensive feats include Choice Band Psychic Terrain Zen Headbutt OHKOing Gyarados, Arcanine Salamence, and even 2HKOing most non fully defensive Porygon2. Unlike the previous two sets, Choice Band Earthquake is able net the OHKOs against many frail Electric, Poison, and some Fire-type Pokemon while dealing a decent chunk against opposing Steel-types. The last moveslot is Bullet Punch so Metagross can use to priority purposes, though remember that Psychic Terrain will render it useless, so use with caution. Hammer Arm can be used to hit opposing Normal-, Dark-, and Steel-types like Porygon2, Snorlax, Kartana, and etc. 

The idea of using Choice Band Metagross is essentially to hit as hard as possible while maintain offensive pressure. One, if not Metagross best partners to pair is Tapu Lele as both receive an increase in STAB Psychic-type moves and can make a "Psychic Spam" team. If anyone does any of the damage calculations for Choice Band Zen Headbutt under Psychic Terrain, most are either 2HKOs dealing about least ~60% damage to almost, if not straight up OHKOs most of the neutral format. Note the Clear Body ability prevents any imposed stat changes, so any "Intimidate" shuffling won't work against Metagross, which will only hit just as consistently hard. The only notable thing to address with this Choice Band Metagross is to definitely consider what move it will "choice lock" itself into until Metagross either switches, gets the Choice Band removed, or faints. 

The EV Spread listed on this set allows Metagross to hit as hard as possible with the Choice Band while outspeed neutral base 70s and potential speed creep mons. Some of the best partners to pair with Choice Band Metagross are predominately offensive Pokemon (Tapu Lele, Pelipper, Golduck, etc) or Trick Room setters (Porygon2, Mimikyum Oranguru, etc) to maintain offensive pressure and speed control prespectively.