So
we're tryna counter mmeta eh.
Some ppl like to think that common mons counter mmeta. I'm here to let the calculator do the talkin let the truth be told.
Lemme also remind you what a counter is: A counter is able to avoid the 2hko pretty safely ON THE SWITCH IN and is able to heavily damage or cripple the target from there.
Without further ado:
First up on the chopping block:
Lando-T
252 Atk Tough Claws Mega Metagross Meteor Mash vs. 252 HP / 240+ Def Landorus-T: 166-196 (43.4 - 51.3%) -- 62.9% chance to 2HKO after Stealth Rock and Leftovers recovery
That's defensive lando-t if you factor in clear body blocking Intimidate. Good shot at 2hko'ing with rocks.
252 Atk Tough Claws Mega Metagross Meteor Mash vs. 0 HP / 24 Def Landorus-T: 226-267 (70.8 - 83.6%) -- guaranteed 2HKO after Stealth Rock
252 Atk Tough Claws Mega Metagross Bullet Punch vs. 0 HP / 24 Def Landorus-T: 102-120 (31.9 - 37.6%) -- guaranteed 3HKO after Stealth Rock
That's scarf lando-t when clear body blocks intimidate. Just dies to meteor+bullet 100% of the time.
Verdict: LANDO-T IS NOT A COUNTER

Next: Rotom-w
252 Atk Tough Claws Mega Metagross Zen Headbutt vs. 248 HP / 216+ Def Rotom-W: 133-157 (43.8 - 51.8%) -- 69.5% chance to 2HKO after Stealth Rock and Leftovers recovery
just gonna get 2hko'd by zen good amount of the time. rotom-w is hardly ever completely healthy anyway.
Verdict: ROTOM-W IS NOT A COUNTER

Next: Slowbro
4 SpA Tough Claws Mega Metagross Grass Knot (80 BP) vs. 252 HP / 0 SpD Slowbro: 192-228 (48.7 - 57.8%) -- guaranteed 2HKO after Stealth Rock and Leftovers recovery
If CM mega bro, it runs a lot of sdef:
4 SpA Tough Claws Mega Metagross Grass Knot (100 BP) vs. 252 HP / 240 SpD Mega Slowbro: 184-218 (46.7 - 55.3%) -- guaranteed 2HKO after Stealth Rock
And then there's thunderpunch:
252 Atk Tough Claws Mega Metagross Thunder Punch vs. 252 HP / 16+ Def Mega Slowbro: 128-152 (32.4 - 38.5%) -- guaranteed 3HKO after Stealth Rock
Tpunch is a little irrelevant heh.
Verdict: (MEGA) SLOWBRO IS NOT A COUNTER.

Next: Ferrothorn
252 Atk Tough Claws Mega Metagross Hammer Arm vs. 252 HP / 88+ Def Ferrothorn: 212-250 (60.2 - 71%) -- guaranteed 2HKO after Stealth Rock and Leftovers recovery
Not only does hammer arm shit on it, sub+power up punch sets up on it, so its really not a fantastic answer.
Verdict: FERROTHORN IS NOT A COUNTER. barely even a check

Next: Defensive Gliscor
252 Atk Tough Claws Mega Metagross Meteor Mash vs. 244 HP / 64+ Def Gliscor: 153-180 (43.4 - 51.1%) -- 4.7% chance to 2HKO after Stealth Rock and Poison Heal
252 Atk Tough Claws Mega Metagross Ice Punch vs. 244 HP / 64+ Def Gliscor: 340-400 (96.5 - 113.6%) -- guaranteed OHKO after Stealth Rock
0 Atk Gliscor Earthquake vs. 0 HP / 0- Def Mega Metagross: 162-192 (53.8 - 63.7%) -- guaranteed 2HKO (assuming mixed)
Gliscor beats it 1v1 if it lacks ice punch, but its forced to roost stall to a good amount of hp, and mmeta could grab an attack boost in that time.
Verdict: GLISCOR IS NOT A COUNTER. Solid check tho.

Next: Mandibuzz
252 Atk Tough Claws Mega Metagross Meteor Mash vs. 248 HP / 136+ Def Mandibuzz: 163-193 (38.5 - 45.6%) -- 81.6% chance to 2HKO after Stealth Rock and Leftovers recovery
252 Atk Tough Claws Mega Metagross Ice Punch vs. 248 HP / 136+ Def Mandibuzz: 182-216 (43 - 51%) -- guaranteed 2HKO after Stealth Rock and Leftovers recovery
0- Atk Mandibuzz Foul Play vs. 0 HP / 0- Def Mega Metagross: 264-312 (87.7 - 103.6%) -- 18.8% chance to OHKO
0- Atk Mandibuzz Foul Play vs. 0 HP / 0 Def Mega Metagross: 236-282 (78.4 - 93.6%) -- guaranteed 2HKO
Reasonably speaking, mandibuzz will pretty much always be mixed, and even defensive variants will get 2hko'd by ice punch after rocks. Even if rocks are off, foul play cannot OHKO mixed variants, so its either forced to roost stall and hope for no attack boost or hope its mixed and get a really high roll.
Verdict: MANDIBUZZ IS NOT A COUNTER. Still great check, but not counter.

Next: Mega Sableye
252 Atk Tough Claws Mega Metagross Meteor Mash vs. 252 HP / 252+ Def Mega Sableye: 132-156 (43.4 - 51.3%) -- 98.8% chance to 2HKO after Stealth Rock
0- Atk Mega Sableye Foul Play vs. 0 HP / 0 Def Mega Metagross: 236-282 (78.4 - 93.6%) -- guaranteed 2HKO
I can understand "hur dur its mega sableye don't calc rocks," but any offensive team worth its shit will find a way to get rocks up vs stall, whether it be skill swap azelf, lum berry SD chomp, or just Landorus. I'm sure there are more I'm forgetting. But that's just my opinion.
So what about when its normal?
252 Atk Tough Claws Mega Metagross Meteor Mash vs. 252 HP / 252+ Def Sableye: 184-217 (60.5 - 71.3%) -- guaranteed 2HKO after Stealth Rock
252 Atk Tough Claws Mega Metagross Bullet Punch vs. 252 HP / 252+ Def Sableye: 82-97 (26.9 - 31.9%) -- 60.5% chance to 3HKO after Stealth Rock
Factoring in stealth rock, mmeta has a great shot at just killing with meteor+bullet. Gets away unscathed.
Verdict: (MEGA) SABLEYE IS NOT A COUNTER.

Next: Skarmory
252 Atk Tough Claws Mega Metagross Thunder Punch vs. 252 HP / 252+ Def Skarmory: 136-160 (40.7 - 47.9%) -- 55.5% chance to 2HKO after Stealth Rock
252 Atk Tough Claws Mega Metagross Hammer Arm vs. 252 HP / 252+ Def Skarmory: 90-106 (26.9 - 31.7%) -- 50.6% chance to 3HKO after Stealth Rock
Well, tpunch isn't the main way ppl beat it. Mmeta with hammer arm will be slower than skarm after a few boosts so when skarm is forced to roost, hammer arm wil hit skarmory as a pure steel-type, 2hko'ing skarm from there. Eventually, mmeta will win.
Verdict: SKARMORY IS NOT A COUNTER.

Next: Bulky Talonflame
252 Atk Tough Claws Mega Metagross Zen Headbutt vs. 248 HP / 252+ Def Talonflame: 168-198 (46.7 - 55.1%) -- 62.5% chance to OHKO after Stealth Rock
252 Atk Tough Claws Mega Metagross Thunder Punch vs. 248 HP / 252+ Def Talonflame: 212-250 (59 - 69.6%) -- guaranteed OHKO after Stealth Rock
usually they're sdef, and the bulky variants end up slower than mmeta, so zen 2hko's even defensive variants without rocks.
Verdict: BULKY TALONFLAME IS NOT A COUNTER.

Next: Zapdos
252 Atk Tough Claws Mega Metagross Zen Headbutt vs. 252 HP / 168+ Def Zapdos: 163-193 (42.4 - 50.2%) -- guaranteed 2HKO after Stealth Rock and Leftovers recovery
252 Atk Tough Claws Mega Metagross Ice Punch vs. 252 HP / 168+ Def Zapdos: 206-244 (53.6 - 63.5%) -- guaranteed 2HKO after Stealth Rock and Leftovers recovery
68 SpA Zapdos Heat Wave vs. 0 HP / 0 SpD Mega Metagross: 162-192 (53.8 - 63.7%) -- guaranteed 2HKO
Gets 2hko'd pretty cleanly.
Verdict: ZAPDOS IS NOT A COUNTER.

Next: Alomomola
252 Atk Tough Claws Mega Metagross Zen Headbutt vs. 248 HP / 252+ Def Alomomola: 156-184 (29.2 - 34.5%) -- 6% chance to 3HKO after Stealth Rock and Leftovers recovery
4 SpA Tough Claws Mega Metagross Grass Knot (60 BP) vs. 248 HP / 8 SpD Alomomola: 222-262 (41.6 - 49.1%) -- 19.1% chance to 2HKO after Stealth Rock and Leftovers recovery
252 Atk Tough Claws Mega Metagross Thunder Punch vs. 248 HP / 252+ Def Alomomola: 198-234 (37.1 - 43.9%) -- guaranteed 3HKO after Stealth Rock and Leftovers recovery
At first glance, it seems to be our first counter. However, Sub+PuP freely sets up on it, and alomomola's scalds do not break the sub. So you can sub 3 times, get to +3, and fire off a +3 zen headbutt at 25%~ while living the final scald.
+3 252 Atk Tough Claws Mega Metagross Zen Headbutt vs. 248 HP / 252+ Def Alomomola: 390-459 (73.1 - 86.1%) -- guaranteed 2HKO after Stealth Rock and Leftovers recovery
0 SpA Alomomola Scald vs. 0 HP / 0 SpD Mega Metagross: 40-48 (13.2 - 15.9%) -- possible 7HKO
mmeta barely beats it.
Verdict: ALOMOMOLA IS NOT A COUNTER. It's a fantastic hard check tho.

Next: Gourgeist-L
252 Atk Tough Claws Mega Metagross Meteor Mash vs. 252 HP / 252+ Def Gourgeist-Large: 135-159 (38.1 - 44.9%) -- guaranteed 3HKO after Stealth Rock and Leftovers recovery
4 Atk Gourgeist-Large Phantom Force vs. 0 HP / 0- Def Mega Metagross: 146-174 (48.5 - 57.8%) -- 93.4% chance to 2HKO
The mothafucking pumpkin is back, with its new and shiny synthesis, its ready to burn and leech seed mmeta to death.
Unfortunately most of them run seed bomb to check subcm keldeo, so sub+pup will still beat gourgiest-L a majority of the time. If you do happen to carry phantom force, u win ignoring a mmash boost on the switch-in.
Verdict: GOURGEIST-L IS A COUNTER.

Next: bulky starmie
4 SpA Tough Claws Mega Metagross Grass Knot (80 BP) vs. 248 HP / 16 SpD Starmie: 180-212 (55.7 - 65.6%) -- guaranteed 2HKO after Stealth Rock and Leftovers recovery
If reflect type:
252 Atk Mega Metagross Earthquake vs. 248 HP / 0 Def Starmie: 272-320 (84.2 - 99%) -- 37.5% chance to OHKO after Stealth Rock
It needs a scald burn basically.
Verdict: STARMIE IS NOT A COUNTER.

Next: Suicune
4 SpA Tough Claws Mega Metagross Grass Knot (100 BP) vs. 252 HP / 0 SpD Suicune: 178-210 (44 - 51.9%) -- 69.5% chance to 2HKO after Stealth Rock and Leftovers recovery
252 Atk Tough Claws Mega Metagross Thunder Punch vs. 252 HP / 252+ Def Suicune: 156-184 (38.6 - 45.5%) -- guaranteed 3HKO after Stealth Rock and Leftovers recovery
It's pretty much done if it switches into gknot, and loses 1v1 to thunderpunch even with CMs under its belt.
Verdict: SUICUNE IS NOT A COUNTER.

Gyarados:
-1 252 Atk Tough Claws Mega Metagross Zen Headbutt vs. 88 HP / 4 Def Gyarados: 153-180 (43.3 - 50.9%) -- guaranteed 2HKO after Stealth Rock and Leftovers recovery
252 Atk Tough Claws Mega Metagross Thunder Punch vs. 0 HP / 4 Def Mega Gyarados: 220-260 (66.4 - 78.5%) -- guaranteed 2HKO after Stealth Rock
4 SpA Tough Claws Mega Metagross Grass Knot (120 BP) vs. 0 HP / 0 SpD Mega Gyarados: 192-228 (58 - 68.8%) -- guaranteed 2HKO after Stealth Rock
252 Atk Tough Claws Mega Metagross Hammer Arm vs. 0 HP / 4 Def Mega Gyarados: 292-344 (88.2 - 103.9%) -- guaranteed OHKO after Stealth Rock
252 Atk Mold Breaker Mega Gyarados Crunch vs. 0 HP / 0- Def Mega Metagross: 236-278 (78.4 - 92.3%) -- guaranteed 2HKO
Gotta play around the mega a little, even then it has options to just kill gyara.
Verdict: GYARADOS IS NOT A COUNTER. One of the best checks offense can manage tho.

Next: Hippowdon
4 SpA Tough Claws Mega Metagross Grass Knot (120 BP) vs. 252 HP / 112 SpD Hippowdon: 272-322 (64.7 - 76.6%) -- guaranteed 2HKO after Stealth Rock and Leftovers recovery
252 Atk Tough Claws Mega Metagross Meteor Mash vs. 252 HP / 144+ Def Hippowdon: 150-177 (35.7 - 42.1%) -- guaranteed 3HKO after Stealth Rock and Leftovers recovery
0 Atk Hippowdon Earthquake vs. 0 HP / 0- Def Mega Metagross: 186-222 (61.7 - 73.7%) -- guaranteed 2HKO
Gknot murders it :[
Verdict: HIPPOWDON IS NOT A COUNTER.

Next: Mega Steelix
252 Atk Tough Claws Mega Metagross Hammer Arm vs. 252 HP / 0 Def Mega Steelix: 150-178 (42.3 - 50.2%) -- 10.9% chance to 2HKO after Stealth Rock
4 SpA Tough Claws Mega Metagross Grass Knot (120 BP) vs. 252 HP / 252+ SpD Mega Steelix: 90-106 (25.4 - 29.9%) -- guaranteed 4HKO after Stealth Rock
4 Atk Sand Force Mega Steelix Earthquake vs. 0 HP / 0- Def Mega Metagross in Sand: 266-314 (88.3 - 104.3%) -- 62.5% chance to OHKO after Stealth Rock
4 Atk Sand Force Mega Steelix Earthquake vs. 0 HP / 0 Def Mega Metagross in Sand: 240-284 (79.7 - 94.3%) -- 6.3% chance to OHKO after Stealth Rock
I'm sure you could ev it to avoid hammer arm 2hko 100% of the time. Sand force eq also has an good chance to OHKO mixed variants. However, jolly variants will usually live even a sand force eq and kill you with the 3rd hammer arm.
Verdict: MEGA STEELIX IS (barely T_T) NOT A COUNTER. Solid check tho.

Next: Mega Aggron
252 Atk Tough Claws Mega Metagross Hammer Arm vs. 252 HP / 16+ Def Filter Mega Aggron: 102-121 (29.6 - 35.1%) -- 87.5% chance to 3HKO after Stealth Rock
0 Atk Mega Aggron Earthquake vs. 0 HP / 0 Def Mega Metagross: 136-162 (45.1 - 53.8%) -- 90.2% chance to 2HKO after Stealth Rock
It wins 1v1 but loses on the switch-in :[
Verdict: MEGA AGGRON IS NOT A COUNTER. Still great check tho.

And finally. The counter. Our lord and saviour:
Mega scizor
252 Atk Tough Claws Mega Metagross Hammer Arm vs. 248 HP / 0+ Def Mega Scizor: 107-127 (31.1 - 37%) -- guaranteed 3HKO after Stealth Rock
Ofc, bulky mega zor can run more defense than that. From there, it can set up SD and even boosted bullet punches will eventually kill. Just pray for no multiple attack boosts. It pretty much needs +4 to break subs with bp tho, if you don't have knock off, so watch out for that.
Verdict: MEGA SCIZOR IS A COUNTER. YAY.

Finally: Keep in mind that these calcs are all JOLLY. I did not take into account ADAMANT meteor mashes from agility sets.