None of the Megas are in the same league as Megaskhan. None of them. The only ones that might be problematic once Megaskhan stops eclipsing them are Mega-Lucario and Mega-Pinsir. Even then, it just doesn't compare. All of the other Megas have multiple easily exploited weaknesses. Lucario and Mawile are susceptible to the omnipresent Earthquake. Mega Lucario is also fairly fragile. Mega-Pinsir is incredible powerful offensively, but has one of the worst defensive type combinations in the game with a 4X Weakness to rock and 2X weaknesses to Ice, Lightning, Fire, and Flying. Seriously, the idea that every Mega Evolution is going to eventually get banned is a joke. Mega-Gengar and Megaskhan simply stand head and shoulders above all the rest.
This made me laugh way harder than it should have. It's true though.
Thank you, I was being facetious though. I haven't heavily used any of the overpowered offensive Megas though, although I did test an all-out attacker (no Rapid Spin) Mega Blastoise (at least it could tank Talonflame's attacks and KO back with Water Pulse which is arguably more reliable than Stone Miss) but I found it is weak to Rotom-W and its Dark Pulse only did about 33% on the specially defensive version.
For Pinsir. A defensive counter would need to resist both Flying and Ground (and some sets use Close Combat) moves. This eliminates most slower grounded Electric, Steel, and Rock types, and since Bug and Grass confer resistance to Ground but also negates an immunity to Flying then they can be OHKO by a + 1 Aerilate Return which can be Swords Dance reduced by Intimidate or attained via Moxie. It would seem that for grounded Pokemon neutral to its STAB and coverage moves and pure defensive bulk is necessary to withstand it. One can run things like Defensive Zapdos, Skarmory, or Rotom-W as slower counters.
Here's a Calc on a bulky Landorus-T:
+1 252 Atk Aerilate Mega Pinsir Return vs. 168 HP / 252 Def Landorus-T: 313-370 (86.7 - 102.4%) -- 93.8% chance to OHKO after Stealth Rock
And Max Physically Defensive Ferrothorn can evade a KO from a neutral STAB flying attack when it is +2 if there are no Spikes or Stealth Rock:
+2 252 Atk Aerilate Mega Pinsir Return vs. 252 HP / 252+ Def Ferrothorn: 295-348 (83.8 - 98.8%) -- guaranteed 2HKO after Leftovers recovery.
+2 252 Atk Aerilate Mega Pinsir Return vs. 252 HP / 252+ Def Ferrothorn: 295-348 (83.8 - 98.8%) -- 37.5% chance to OHKO after Stealth Rock
And for Bulky Offensive Zygarde (Max HP Evs on base 108 and 122 base def)
+2 252 Atk Aerilate Mega Pinsir Return vs. 252 HP / 0 Def Zygarde: 420-495 (100 - 117.8%) -- guaranteed OHKO
Its initial 85 base speed is a debilitating when it comes in but it has Quick Attack to remedy it. It is weak on the special side though.
A strong STAB attack from a faster Pokemon can kill even the defensive variants though.
252 SpA Thundurus Thunderbolt vs. 252 HP / 0 SpD Mega Pinsir: 326-386 (97.6 - 115.5%) -- 87.5% chance to OHKO
although coverage moves are unreliable and need boosting items even when Rocks are up:
252 SpA Manaphy Ice Beam vs. 252 HP / 0 SpD Mega Pinsir: 188-222 (56.2 - 66.4%) -- guaranteed 2HKO
252 SpA Manaphy Ice Beam vs. 0 HP / 0 SpD Mega Pinsir: 188-222 (69.3 - 81.9%) -- guaranteed 2HKO
(Manaphy, a base 100, has to run positive nature speed in order to land this hit during the turn of Mega Evolution although it can be killed by Quick Attack if its HP is low enough, and that might even trigger a free Moxie boost)
the physical side is much harder:
252 Atk Life Orb Mamoswine Ice Shard vs. 0 HP / 0 Def Mega Pinsir: 148-174 (54.6 - 64.2%) -- guaranteed 2HKO
252+ Atk Choice Band Mamoswine Icicle Crash vs. 0 HP / 0 Def Mega Pinsir: 390-462 (143.9 - 170.4%) -- guaranteed OHKO
Strong priority from power Pokemon with a boosting item.
252 Atk Mamoswine Icicle Crash vs. 0 HP / 0 Def Mega Pinsir: 236-282 (87 - 104%) -- 25% chance to OHKO
A neutral nature with no boosting item fails to guarantee a KO with no Rocks, and unlike the special example a positive nature or boosting item is required. So I am surprised no one is using Substitute on this as a countermeasure against those who may try to burn or paralyze it while it is setting up.
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Well, due to Talonflame, Weakness Policy Dragonite, the Mega Charizards, and Mega-Pinsir, perhaps Gen 6 would be known for the Rock Wars instead of the Weather Wars.