Rest
In
Pyukumuku
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As for Z moves, I'd like to look at Tapu Koko, Necrozma, Dragonite, and Porygon Z.
Tapu Koko (Timid): 289 special attack, 1.5x boost from Electric Terrain, base 185 Gigavolt Havoc with perfect accuracy.
Let's compare that to the premier electric type of the tier, Specs Magnezone (Modest): 394 spA, 1.5x boost from Specs, base 90 Thunderbolt with perfect accuracy.
252+ SpA Choice Specs Magnezone Thunderbolt vs. 0 HP / 0 SpD Eevee: 345-
406 (137.4 - 161.7%) -- guaranteed OHKO
252 SpA Tapu Koko Gigavolt Havoc vs. 0 HP / 0 SpD Eevee: 346-
408 (137.8 - 162.5%) -- guaranteed OHKO
Electrium Z Tapu Koko is almost exactly on par with Magnezone. Now, let's consider the other differences between the two. Tapu Koko has speed (base 130), whereas Zone has Sturdy. This means both pokemon generally get to fire off one hit. Both pokemon have secondary STAB in Flash Cannon and Dazzling Gleam. Both often run HP Fire or Ice. Magnezone's Flash Cannon hits considerably harder than Koko's Gleam, but Tapu Koko has something Magnezone doesn't: the freedom to switch moves. Tapu Koko can run Taunt, Sub, Nature's Madness, etc. to deal with situations that Koko would otherwise find unfavorable.
Overall, these two electric types are almost exactly even in terms of viability. Tapu Koko alone is not powerful enough to warrant a ban on Z moves.
Necrozma is a unique case since he's a bulky psychic type that can set up with Calm Mind and Moonlight before firing off a 200 base power Shattered Psyche.
There aren't many offensive Psychic types in 1v1, but we can compare it to Specs Meloetta, Mega Gardevoir, and Mega Zam who all play a somewhat similar role.
252+ SpA Necrozma Shattered Psyche vs. 0 HP / 0 SpD Eevee: 501-
591 (199.6 - 235.4%) -- guaranteed OHKO
+1 252+ SpA Necrozma Shattered Psyche vs. 0 HP / 0 SpD Eevee: 753-
886 (300 - 352.9%) -- guaranteed OHKO
252+ SpA Choice Specs Meloetta Psychic vs. 0 HP / 0 SpD Eevee: 340-
402 (135.4 - 160.1%) -- guaranteed OHKO
252+ SpA Gardevoir-Mega Psychic vs. 0 HP / 0 SpD Eevee: 274-
324 (109.1 - 129%) -- guaranteed OHKO
252+ SpA Alakazam-Mega Psychic vs. 0 HP / 0 SpD Eevee: 288-
339 (114.7 - 135%) -- guaranteed OHKO
Necrozma blows every other Psychic move in the tier out of the water. Unboosted Shattered Psyche does nearly 1.5x that of Modest Specs Meloetta's Psychic. If you can get a CM off first (and you're usually able to do so), Shattered Psyche obliterates everything that isn't a Dark type.
This is crazy enough damage for a z move ban to be considered, but by itself isn't broken. Necrozma can be built around if it is the only Z move user.
Dragonite has always been an A tier mon. It sometimes runs DD+Lum/WP, but most Dragonite use Choice Band. How much better is DD Dragonium Z Dnite?
252+ Atk Choice Band Dragonite Outrage vs. 0 HP / 0 Def Eevee: 571-
673 (227.4 - 268.1%) -- guaranteed OHKO
252+ Atk Dragonite Devastating Drake vs. 0 HP / 0 Def Eevee: 603-
711 (240.2 - 283.2%) -- guaranteed OHKO
+1 252+ Atk Dragonite Devastating Drake vs. 0 HP / 0 Def Eevee: 904-
1065 (360.1 - 424.3%) -- guaranteed OHKO
Without boosting, Dragonium Z already hits significantly harder than Choice Band. With a single DD (which Dnite can almost always get off due to Multiscale), you deal nearly twice the damage of banded Dnite,
without having to lock yourself into a move. That means you can run coverage / Espeed to pick off Sturdy mons and the like. Frankly, Dragonium Z is the only way to run Dnite in the current meta, unless you're so afraid of status that you
need to run Lum DD.
This is another huge point in favor of banning Z moves. Pokemon that were
already really good become waaay better.
I'm not going to run any calcs on Porygon Z, because I don't have to. The fact that Porygon Z can viably run:
-Timid Scarf
-Fast Modest Scarf
-Defensive Modest Scarf
-Fast Timid Specs
-Defensive Modest Specs
-Fast Custap Berry
-Defensive Custap Berry
-Normalium Z Hyper Beam (200 BP, no recharge)
-Normalium Z Conversion
-Ghostium Z Conversion
-Icium Z Conversion
-Electrium Z Conversion
ALL with their own checks and counters, put's Mega Gyarados's famous "versatility" to shame, especially when you consider the absurd amount of move choices Pory has in addition to it's signature Hyper Beam: Thunderbolt, Ice Beam, Dark Pulse, Shadow Ball, Psychic/Psyshock, Uproar, Trick, and (non-Z) Conversion.
Porygon Z has easily become the most difficult pokemon to prepare for in the entire tier of 1v1. It leads the list of pokemon that simply have
too many god damn sets they can use to startling effectiveness, thanks to the introduction of Z moves. This is the real issue with Z stones - the unpredictability of movesets. All of a sudden we are seeing Crustle, Greninja, Garchomp, and many others deal
way too much damage out of nowhere, with zero side effects (no recoil, no recharge, no move locking). They even break through Protect. Z Stones make already viable pokemon god-tier, and let them obliterate the pokemon that can't properly utilize these unpredictable and overpowered moves.
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BAN Z STONES. (but stop using Tapu Koko as a reason, he's actually pretty balanced)