I ll keep this long, i have not made a long post in quite a while so i guess its time again.
The big addition US/UM gave us is Photon Geyser. I agree with Morogrim on that topic.
MMX was always hindered by the lack of a good Psychic STAB which can outperform MMY Psystrike.
It got it with Photon Geyser, an intelligent defensive targeting move with Mold Breaker effect.
Too bad both M-Medicham and M-Gallade share MMX typing otherwise they could have made an entrance into BH.
Giratina is not an answer to MMX anymore, Choice Band can 2HKO without Adaptability.
252 Atk Choice Band Mewtwo-Mega-X Photon Geyser vs. 252 HP / 252 Def Giratina: 228-268 (45.2 - 53.1%) -- 31.6% chance to 2HKO
This is big as Giratina used to be a relatively safe switch in.
My favorite Item for MMX is Mind Plate, it gives a solid chance to 2HKO Giratina.
252 Atk Mind Plate Adaptability Mewtwo-Mega-X Photon Geyser vs. 252 HP / 252 Def Giratina: 244-288 (48.4 - 57.1%) -- 91.8% chance to 2HKO
And the damage is similar to non- Adaptability Choice Band making Band bluffing possible.
Tough Claws is an option to boost Spectral Thief and U-Turn, it has a good chance to 1HKO other MMX on switch in.
252 Atk Choice Band Tough Claws Mewtwo-Mega-X Spectral Thief vs. 252 HP / 252 Def Mewtwo-Mega-X: 404-476 (97.1 - 114.4%) -- 81.3% chance to OHKO
You can trade Close Combat for V-Create with this ability, you still 1HKO Registeel and switch your counterspectrum a bit.
Greninja-Ash is the only Dark type faster than MMX so it can switch in, Encore, Substall, Set Up or revengekill,
252 Atk Life Orb Adaptability Mewtwo-Mega-X Mach Punch vs. 252 HP / 252 Def Greninja-Ash: 312-369 (89.6 - 106%) -- 37.5% chance to OHKO
Rocks up means only Fur Coat Greninja is safe.
Another check is Yveltal, which its is forced to have Fur Coat and still cant switch in with rocks up.
252 Atk Adaptability Mewtwo-Mega-X Close Combat vs. 252 HP / 252+ Def Yveltal: 260-308 (57 - 67.5%) -- guaranteed 2HKO
252 Atk Fist Plate Adaptability Mewtwo-Mega-X Close Combat vs. 252 HP / 252+ Def Yveltal: 312-368 (68.4 - 80.7%) -- 50% to OHKO with SR
252 Atk Life Orb Adaptability Mewtwo-Mega-X Close Combat vs. 252 HP / 252+ Def Yveltal: 338-400 (74.1 - 87.7%) -- 93.8% to OHKO with SR
252 Atk Choice Band Adaptability Mewtwo-Mega-X Close Combat vs. 252 HP / 252+ Def Yveltal: 390-460 (85.5 - 100.8%) -- 6.3% chance to OHKO
M-Sableye is the closest thing to a counter as it is immune to both of MMX STABs
252 Atk Life Orb Tough Claws Mewtwo-Mega-X Sunsteel Strike vs. 252 HP / 252+ Def Sableye-Mega: 152-179 (50 - 58.8%) -- guaranteed 2HKO
252 Atk Life Orb Tough Claws Mewtwo-Mega-X V-create vs. 252 HP / 252+ Def Fur Coat Sableye-Mega: 136-161 (44.7 - 52.9%) -- 28.1% to 2HKO
It can still get 2HKoed by Sunsteel or V-Create in Sun, but pretty much the perfect Imposterproofer.
Ironically MMY is the most practical check, it resits both STABs, has neutral STAB itself and outspeeds.
252 SpA Mewtwo-Mega-Y Psycho Boost vs. -1 252 HP / 252 SpD Mewtwo-Mega-X: 367-433 (88.2 - 104%) -- 25% chance to OHKO
Fleur Cannon is a 1HKO at -1, otherwise Spec and/or Terrain needed.
Another popular check is M-Rayquaza, Fake Speed can be blocked by Psychic Terrain so only Scarf Aerilite Boomburst works guaranted,
252 Atk Rayquaza-Mega Dragon Ascent vs. 252 HP / 252 Def Mewtwo-Mega-X: 396-468 (95.1 - 112.5%) -- 68.8% chance to OHKO
Using Terrain Scarf M-Ray has a chance to not 1HKO, nevertheless M-Ray has typing advantage and MMX has to respect that.
The Imposter stays a valid option to check MMX. The MMX must be really careful with set up moves if it runs Photon Geyser.
252 Atk Choice Band Adaptability Mewtwo-Mega-X Photon Geyser vs. +1 252 HP / 252 Def Mewtwo-Mega-X in Psychic Terrain: 346-408 (49.1 - 57.9%) -- 97.7% chance to 2HKO
252 Atk Adaptability Mewtwo-Mega-X Photon Geyser vs. 252 HP / 252 Def Mewtwo-Mega-X in Psychic Terrain: 346-408 (83.1 - 98%) -- guaranteed 2HKO
In this scenario Choice Band is a practical way to get an important Power edge which most Imposters are missing.
I think any ability other than Psychic Terrain, Adaptability or Touch Claws is pointless on MMX now.
It gets all the power it needs without being stopped by Prankster Haze like all those set upper sets.
Its most effective moveset is Close Combat + Photon Geyser, Spectral Thief for coverage and one move of choice.
The prediction factor is huge, you will have to click Photon Geyser on Dark Types if you predict them to switch.
I would still judge MMY as superior to MMX, it can nuke slightly harder, its easier to imposterproof and it has better 1on1 matchups.
MMY has just the right attack/defense ratio to 2HKO other MMY with V-Create and avoiding a speedtie with M-Gengar helps a lot.
MMX is close to its power and because of its dual typing it changes the meta more than MMY.
Strong dual types tend to get banned quicker, take a look at P-Don compared with P-Kyogre.
The big addition US/UM gave us is Photon Geyser. I agree with Morogrim on that topic.
MMX was always hindered by the lack of a good Psychic STAB which can outperform MMY Psystrike.
It got it with Photon Geyser, an intelligent defensive targeting move with Mold Breaker effect.
Too bad both M-Medicham and M-Gallade share MMX typing otherwise they could have made an entrance into BH.
Giratina is not an answer to MMX anymore, Choice Band can 2HKO without Adaptability.
252 Atk Choice Band Mewtwo-Mega-X Photon Geyser vs. 252 HP / 252 Def Giratina: 228-268 (45.2 - 53.1%) -- 31.6% chance to 2HKO
This is big as Giratina used to be a relatively safe switch in.
My favorite Item for MMX is Mind Plate, it gives a solid chance to 2HKO Giratina.
252 Atk Mind Plate Adaptability Mewtwo-Mega-X Photon Geyser vs. 252 HP / 252 Def Giratina: 244-288 (48.4 - 57.1%) -- 91.8% chance to 2HKO
And the damage is similar to non- Adaptability Choice Band making Band bluffing possible.
Tough Claws is an option to boost Spectral Thief and U-Turn, it has a good chance to 1HKO other MMX on switch in.
252 Atk Choice Band Tough Claws Mewtwo-Mega-X Spectral Thief vs. 252 HP / 252 Def Mewtwo-Mega-X: 404-476 (97.1 - 114.4%) -- 81.3% chance to OHKO
You can trade Close Combat for V-Create with this ability, you still 1HKO Registeel and switch your counterspectrum a bit.
Greninja-Ash is the only Dark type faster than MMX so it can switch in, Encore, Substall, Set Up or revengekill,
252 Atk Life Orb Adaptability Mewtwo-Mega-X Mach Punch vs. 252 HP / 252 Def Greninja-Ash: 312-369 (89.6 - 106%) -- 37.5% chance to OHKO
Rocks up means only Fur Coat Greninja is safe.
Another check is Yveltal, which its is forced to have Fur Coat and still cant switch in with rocks up.
252 Atk Adaptability Mewtwo-Mega-X Close Combat vs. 252 HP / 252+ Def Yveltal: 260-308 (57 - 67.5%) -- guaranteed 2HKO
252 Atk Fist Plate Adaptability Mewtwo-Mega-X Close Combat vs. 252 HP / 252+ Def Yveltal: 312-368 (68.4 - 80.7%) -- 50% to OHKO with SR
252 Atk Life Orb Adaptability Mewtwo-Mega-X Close Combat vs. 252 HP / 252+ Def Yveltal: 338-400 (74.1 - 87.7%) -- 93.8% to OHKO with SR
252 Atk Choice Band Adaptability Mewtwo-Mega-X Close Combat vs. 252 HP / 252+ Def Yveltal: 390-460 (85.5 - 100.8%) -- 6.3% chance to OHKO
M-Sableye is the closest thing to a counter as it is immune to both of MMX STABs
252 Atk Life Orb Tough Claws Mewtwo-Mega-X Sunsteel Strike vs. 252 HP / 252+ Def Sableye-Mega: 152-179 (50 - 58.8%) -- guaranteed 2HKO
252 Atk Life Orb Tough Claws Mewtwo-Mega-X V-create vs. 252 HP / 252+ Def Fur Coat Sableye-Mega: 136-161 (44.7 - 52.9%) -- 28.1% to 2HKO
It can still get 2HKoed by Sunsteel or V-Create in Sun, but pretty much the perfect Imposterproofer.
Ironically MMY is the most practical check, it resits both STABs, has neutral STAB itself and outspeeds.
252 SpA Mewtwo-Mega-Y Psycho Boost vs. -1 252 HP / 252 SpD Mewtwo-Mega-X: 367-433 (88.2 - 104%) -- 25% chance to OHKO
Fleur Cannon is a 1HKO at -1, otherwise Spec and/or Terrain needed.
Another popular check is M-Rayquaza, Fake Speed can be blocked by Psychic Terrain so only Scarf Aerilite Boomburst works guaranted,
252 Atk Rayquaza-Mega Dragon Ascent vs. 252 HP / 252 Def Mewtwo-Mega-X: 396-468 (95.1 - 112.5%) -- 68.8% chance to OHKO
Using Terrain Scarf M-Ray has a chance to not 1HKO, nevertheless M-Ray has typing advantage and MMX has to respect that.
The Imposter stays a valid option to check MMX. The MMX must be really careful with set up moves if it runs Photon Geyser.
252 Atk Choice Band Adaptability Mewtwo-Mega-X Photon Geyser vs. +1 252 HP / 252 Def Mewtwo-Mega-X in Psychic Terrain: 346-408 (49.1 - 57.9%) -- 97.7% chance to 2HKO
252 Atk Adaptability Mewtwo-Mega-X Photon Geyser vs. 252 HP / 252 Def Mewtwo-Mega-X in Psychic Terrain: 346-408 (83.1 - 98%) -- guaranteed 2HKO
In this scenario Choice Band is a practical way to get an important Power edge which most Imposters are missing.
I think any ability other than Psychic Terrain, Adaptability or Touch Claws is pointless on MMX now.
It gets all the power it needs without being stopped by Prankster Haze like all those set upper sets.
Its most effective moveset is Close Combat + Photon Geyser, Spectral Thief for coverage and one move of choice.
The prediction factor is huge, you will have to click Photon Geyser on Dark Types if you predict them to switch.
I would still judge MMY as superior to MMX, it can nuke slightly harder, its easier to imposterproof and it has better 1on1 matchups.
MMY has just the right attack/defense ratio to 2HKO other MMY with V-Create and avoiding a speedtie with M-Gengar helps a lot.
MMX is close to its power and because of its dual typing it changes the meta more than MMY.
Strong dual types tend to get banned quicker, take a look at P-Don compared with P-Kyogre.