Defensive Landorus-T usually lacks the capabilities to threaten Mega Heracross, as Toxic is pretty much non-existent on it, unlike in late XY.This is my first post so apologies in advance, I just wanted to talk about Mhera.
A- --> A- Imo
The main arguments against this are things such as the fact that it is destroyed by sableye, is slow and generally isn't favoured in a meta where base 110 speed is seen as a benchmark of sorts. It is also completely destroyed by almost all talonflame variants and Maltaria in particular. However it still functions as an amazing threat to stall and can seriously threaten teams which rely on specific leads such as sash breloom (and sash mamo), which is slower if you run jolly with 216 investment, albeit I prefer to run. It also allows you to outspeed adamant Mgyara and slower mew/manaphy and heatran variants.
This combined with its monstrous 185 attack stat actually make it a very competent threat to almost all defensive pokemon aside from gliscor and Defensive lando-t.
A few calcs
252 Atk Mega Heracross Pin Missile (5 hits) vs. 252 HP / 252+ Def Mega Sableye: 165-200 (54.2 - 65.7%) -- guaranteed 2HKO, so provided you get some form of initial damage you can deal with sableye, or
252+ Atk Mega Heracross Pin Missile (5 hits) vs. 252 HP / 252+ Def Mega Sableye: 185-225 (60.8 - 74%) -- guaranteed 2HKO
252 Atk Mega Heracross Pin Missile (5 hits) vs. 0 HP / 4 Def Breloom: 320-380 (122.6 - 145.5%) -- guaranteed OHKO
252 Atk Mega Heracross Close Combat vs. 4 HP / 0 Def Bisharp: 1024-1212 (376.4 - 445.5%) -- guaranteed OHKO, best bisharp can do is Iron head (41-48%), and it can't outspeed jolly even if 252
252 Atk Mega Heracross Pin Missile (5 hits) vs. 252 HP / 252+ Def Mega Slowbro: 250-310 (63.4 - 78.6%) -- guaranteed 2HKO
252 Atk Mega Heracross Rock Blast/Bullet Seed (5 hits) vs. 252 HP / 160 Def Clefable: 195-230 (49.4 - 58.3%) -- approx. 61.7% chance to 2HKO after Leftovers recovery
Pre mega hera also outspeeds metagross so 252 Atk Mega Heracross Pin Missile (5 hits) vs. 0 HP / 0- Def Mega Metagross: 210-255 (69.7 - 84.7%) -- guaranteed 2HKO (Albeit Zen headbutt still OHKO's you but You will have heavily crippled a threat)
252 Atk Mega Heracross Rock Blast (5 hits) vs. 252 HP / 168+ Def Zapdos: 320-380 (83.3 - 98.9%) -- guaranteed 2HKO after Leftovers recovery
252 Atk Mega Heracross Pin Missile (5 hits) vs. 252 HP / 104+ Def Mew: 450-540 (111.3 - 133.6%) -- guaranteed OHKO
252 Atk Mega Heracross Bullet Seed (5 hits) vs. 96 HP / 0 Def Manaphy: 360-430 (98.6 - 117.8%) -- approx. 93.8% chance to OHKO
252 Atk Mega Heracross Pin Missile (5 hits) vs. 252 HP / 252+ Def Chesnaught: 170-210 (44.7 - 55.2%) -- approx. 5.1% chance to 2HKO after Leftovers recovery
I know a lot of these are quite cherrypicked but it shows just how much of the pokemon in the stall playstyle Mhera can 2HKO reasonably consistently
Of course I am not saying Mhera should be A/A+ rank or anything, I just haven't seen anybody argue for It and being a player who has built a team around Mhera (and gotten to 1750+ with it) I feel it would be bad not to offer a different perspective.
These cherry-picked calcs really do not solidify your argument. Mega Heracross still has to deal with Gliscor and Unaware Clefable, and while it threatens a lot of Pokemon seen on stall, it really suffers from a large amount of opportunity cost and mediocre speed that really weigh it down. You seriously cannot say Mega Heracross is on par with Celebi, Starmie, and Jirachi, which usually are much more flexible than Mega Heracross and not be a sitting duck to be forced out by any form of offensive pressure. Sure, Mega Hera might be comparable to Mamoswine and Terrakion, but I really do not think that Mega Heracross' pros outweigh its cons enough to be A- Rank.