Firium and Groundium are both complete unsets so they won't be counted
What is included:
Z-Outrage
Z-Fly
Z-Hurricane
Choice Scarf
Choice Band

S Rank
Charizard-Mega-X 50/50 vs dd dragonium and Scarf is another 50/50 if zard's even speed crept to outspeed after fc/dd
Dragonite bruh
Gyarados-Mega Counter 1

S- Rank
Magearna Band

A+ Rank
Charizard-Mega-Y Lol no
Greninja Scarf/Specially Defensive
Metagross-Mega Counter 2
Porygon-Z Lol no
Tapu Lele Choice Scarf (this one it actually wins rather consistently, adamant can )

A Rank
Gardevoir-Mega actually does a pretty good job. Only problem is rng reliant on Choice Scarf, but I'll give it anyway. Counter 3
Meloetta Any physical
Mew As long as you git gud and predict set Counter 4
Slowbro-Mega You can actually mess up against Z-Hurricane and still be fine Counter 5
Zeraora BU Electrium (at zard x bulk) gets rolled on really hard by Z-Outrage. Life Orb dies to Jolly Scarf. Dragonium is unset, not even going to put it in the counter counter (heh heh).
Zygarde-Complete Everything but Z-Fly and Choice Scarf, so Lol no

A- Rank
Aegislash Lol no
Kommo-o Lol no
Landorus-Therian Z-Hurricane
Magnezone Z-Outrage
Primarina Z-Fly
Tapu Fini Z-Fly
Victini Need Icium, which is hard unsetting but I"ll be nice and Counter 6

B+ Rank
Altaria-Mega Git gud and predict (but you need Confide for Z-Hurricane) Counter 7
Celesteela Choice Band
Clefable Git Gud and predict Counter 8
Donphan Z-Hurricane beats bulky BandPhan
Garchomp Lol no
Genesect Everything but Z-Outrage, so Lol no
Heatran Everything but Z-Hurricane
Jumpluff Just don't miss, Counter 9
Mawile-Mega (You actually need Metal Burst or Sucker Punch, otherwise band would win) Counter 10
Sableye-Mega Just git gud and predict the set Counter 11
Serperior Lol no
Togekiss T Wave Charm things, except you don't have to predict set on this one Counter 12
Tyranitar-Mega Choice Band or Choice Scarf
Venusaur-Mega Flyinium

B Rank
Aggron-Mega Metal Burst, Counter 13
Blaziken-Mega Lol no
Crustle Z-Hurricane always wins (Jolly needs five rock blast hits for a 31.3% chance to KO)
Ferrothorn Z-Hurricane, Choice Band, Choice Scarf
Haxorus Mr. My One Job is Killing Dragonite Counter 14
Hoopa-Unbound Lol no
Incineroar Z-Hurricane
Jirachi needs three flinch to beat Choice Band
Kartana Lol no
Krookodile Lol no
Lopunny-Mega Jolly Choice Scarf
Magneton Z-Outrage
Naganadel Lol no
Necrozma Just git gud and predict set, Counter 15
Pheromosa Need Icium, which is hard unsetting but I"ll be nice and Counter 16
Porygon2 Actually just wins legitimately, even without Trace Counter 17
Registeel Choice Band
Sawk Lol no
Scizor-Mega Z-Hurricane or Choice Band or Choice Scarf
Type: Null Z-Outrage
Volcarona Lol no
Whimsicott Z-Hurricane

B- Rank
Archeops Choice Scarf
Carracosta Lol no
Deoxys-S 50/50 Z-Outrage
Diancie-Mega Choice Band or Choice Scarf
Golem 50/50 with Sucker Punch and Extreme Speed and Counter (need to be Rockium tho)
Kyurem Icium can actually just straight up face tank Choice Scarf, so it's just the speed war, and I'll count it Counter 18
Latias-Mega Lol no
Manaphy Lol no
Medicham-Mega Choice Scarf
Pinsir-Mega Lol no
Swampert-Mega Just get the Yawn roll idiot, Counter 19
Tapu Bulu Lol no
Vivillon Choice Scarf or Choice Band Extreme Speed
Zapdos Lol no

So let's look at the counters!
Cheese strats: Mega Gyarados, Mega Gardevoir, Mega Aggron, Haxorus, Mega Swampert (I'm not counting Mega Maw): 5 mons
Hard Stall mons no set predict needed: Mega Slowbro, Jumpluff, Charm Togekiss: 3 mons
Hard Stall mons + git gud and predict set: Kew, Mega Altaria, Clefable, Mega Sableye, Necrozma: 5 mons
Straight Unsets: Icium Victini, Icium Pheromosa, Dragonium Zeraora sux: 2 mons
Actually win legit: Ice Punch Mega Metagross, Mega Mawile, Porygon2, fast(er) Icium Kyurem: 4 mons