Question

What are the best Z users in Gen 7 OU?

Answer 1

Here are two boxes containing the most common Z-Move users. For the sake of brevity, only Pokemon that are OU by tiering or at least B rank on VR have been listed. There are dozens of more niche Z-move users not listed here. Refer to smogon analyses to see why they are good. Note that Azumarill, Blacephalon and Keldeo are far worse than other OU Pokemon and are only OU due to being spammed incessantly on low ladder. All movesets and EV spreads are taken from Smogon analyses.
https://pokepast.es/34efd5834b54f7e1
https://pokepast.es/bc8c7de82b96a1c2

Comments

This answer was posted as the other two were outdated and/or incomplete.

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The OU tier hasn't updated since 2019, so it's probably better to list Pokemon from the top of the OU viability rankings. This answer currently misses Weavile and Reuniclus, which are considered "viable" now but were not OU at the end of gen 7.

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Weavile doesn't usually run Z Moves, but I will add Reuniclus. I think the Z Move Weaviles are too niche to add.

Answer 2

Z-Moves in OU by Smogon

Flying

Flyinium Z Landorus-Therian @ Flyinium Z
Ability: Intimidate
EVs: 252 Atk / 4 Def / 252 Spe
Adamant Nature
- Earthquake
- Fly
- Smack Down
- Stealth Rock

Ice

Icium Z Kyurem-Black @ Icium Z
Ability: Teravolt
EVs: 252 Atk / 4 SpA / 252 Spe
Naive Nature
- Freeze Shock
- Fusion Bolt
- Ice Beam
- Earth Power

Grass

Grassium Z Tapu Bulu @ Grassium Z
Ability: Grassy Surge
EVs: 252 Atk / 4 Def / 252 Spe
Adamant Nature
- Swords Dance
- Wood Hammer
- Horn Leech
- Superpower

Fairy

Fairium Z Magearna @ Fairium Z
Ability: Soul-Heart
EVs: 252 HP / 4 Def / 252 SpA
IVs: 0 Spe
Quiet Nature
- Trick Room
- Fleur Cannon
- Focus Blast
- Thunderbolt

Dragon

Dragonium Z Zygarde @ Dragonium Z
Ability: Aura Break
EVs: 4 HP / 252 Atk / 252 Spe
Adamant Nature
- Thousand Arrows
- Outrage
- Extreme Speed
- Dragon Dance

Ground

Groundium Z Garchomp @ Groundium Z
Ability: Rough Skin
EVs: 4 HP / 252 Atk / 252 Spe
Jolly Nature
- Earthquake
- Swords Dance
- Dragon Claw
- Fire Fang

Ghost

Ghostium Z Gengar @ Ghostium Z
Ability: Cursed Body
EVs: 4 HP / 252 SpA / 252 Spe
Timid Nature
- Shadow Ball
- Sludge Wave
- Focus Blast
- Taunt

Electric

Zapdos @ Electrium Z
Ability: Static
EVs: 252 SpA / 4 SpD / 252 Spe
Modest Nature
IVs: 0 Atk
- Agility
- Thunderbolt
- Heat Wave
- Hidden Power Ice

Dark

Bisharp @ Darkinium Z
Ability: Defiant
EVs: 252 Atk / 4 SpD / 252 Spe
Adamant Nature
- Knock Off
- Iron Head
- Sucker Punch
- Swords Dance

Water

Omastar @ Waterium Z
Ability: Swift Swim
EVs: 252 SpA / 4 SpD / 252 Spe
Modest Nature
IVs: 0 Atk
- Shell Smash
- Hydro Pump
- Earth Power
- Ice Beam

Fire

Volcarona @ Firium Z
Ability: Flame Body
EVs: 252 SpA / 4 SpD / 252 Spe
Timid Nature
- Quiver Dance
- Fire Blast
- Giga Drain
- Hidden Power Ground

Comments

What criteria did you chose these with? Zygarde isn't allowed in OU, Omanstar is unviable and Groundium Z Chomp is very niche. There's a lot of bad Z-Move users included and a lot if good ones left out.

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They come straight from the Smogon article linked in the answer. There are archives for it going back to Aug '17, so it's really outdated.

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That article is outdated, i'll post an updated list.

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thank you @cdiijk16 for the updated list

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Thanks, you're welcome.