Because Rotom-W has a higher base speed, and just as bad as it is to run near max speed Heracross at the sacrifice of bulk, its just as doable with Rotom-W, and Rotom-W needs less investment at every point than Heracross because its base speed is higher. Also as I said, Rotom-W stays in on Heracross, Burns it, and wins the fight.
Okay, last thing about this:
- Mega Heracross typically runs 40 HP/252+ Atk/216 Spe, making it 240 Speed. Even so, Rotom-W only runs 44 EVs, giving it 219 Spe. Use a goddamn calculator before you make bullshit assumptions, please.
- Rotom-W can't switch into Mega Heracross without being outsped and 2HKOed. It's also
not coming in on Swords Dance variants unless you want a wrecked washing machine, which is nice, I guess.
Now, time to move on to a nomination:
Rhyperior (B-) -> B | Rhyperior is pretty underrated these days. It's pretty much the most solid Flying Spam check in the game, thanks to its typing and massive 115/130 physical defenses. In terms of offense, Rhyperior's Ground/Rock-type gives it the coveted EdgeQuake and it has just about every tool it needs to screw some other dangerous things over, such as hitting Slowbro and other Psychic-types with Megahorn, opposing Rock- and Ground-types with Aqua Tail and Gliscor and the Landorus with Ice Punch. With this brilliant offensive coverage comes base 140 Attack, which is pretty damn impressive. From using a physically defensive set to an Assault Vest attacker to a Choice Bander, Rhyperior can pull it off pretty well given the right support.
Speaking of which, Rhyperior's gonna need some support if you're using it, as its base 55 Special Defense really offsets its amazing physical bulk. Base 40 Speed isn't terribly impressive, either, meaning Rhyperior's gonna have to take a hit before it moves. It also has six weaknesses, two of which are double (Water and Grass); the fact that most of these weaknesses are quite common doesn't particularly help, either. It also lacks reliable recovery, which Hippowdon and Gliscor do possess. Fortunately, there are ways to help Rhyperior deal with these flaws: Pokémon that can remove dangerous special attackers (AV Azumarill comes to mind) and Pokémon that can deal with slower wallbreakers that outpace Rhyperior (Azumarill once again, but moreso Talonflame) come to mind. Its low Speed can be turned to one's favor by utilizing Rhyperior on a Trick Room team, on which it's a brilliant Pokémon. In short, Rhyperior has some definite advantages that can easily be employed on teams in the current metagame, especially with the rise of Flying Spam.
Rhyperior for B.