Hydreigon(Yes I know Insurgence has one, this is just mine) Before: HP: 92 Atk: 105 Def: 90 SpA: 125 SpD: 90 Spe: 98 Total: 600 After: HP: 92 Atk: 105 Def: 105 SpA: 165 SpD: 105 Spe: 128 Total: 700 After Mega evolving, Hydreigon turns into a scarily fast hard-hitting special attacker, boasting a base 128 speed outspeeding common threats like Mega Salamence and Mega Rayquaza. Aside that, it also has very solid Defense stats which, given its mediocre typing, is very much needed. Its high Special Attack stat allows it to maintain Life Orb level Hydreigon hitting power without the recoil.
Mega Hydreigon moveset: Hydreigon-Mega @ Hydreigonite
EVs: 4 HP / 252 SpA / 252 Spe
Draco Meteor Dark Pulse Fire Blast/Flash Cannon Draconic Roar
Draco Meteor is a high base power STAB move which hits hard on anything that doesn’t resist it such as Kyurem-White or Mega Mewtwo X. Dark Pulse is another STAB move to hit Steel types that resist Draco Meteor but are hit super effectively by Dark type moves, such as Solgaleo or Mega Metagross. Fire Blast can hit Steel types that resist both of these moves, like Mega Lucario and Mega Mawile, or provide a reliable way of hitting Solgaleo and Metagross on a non-Dark Pulse set. Fire Blast can also be used to hit bulky Grass types hard without resorting to Draco Meteor for the Special Attack drop. Finally we have Draconic Roar(I created a new move, +1 SpA and +1 Spe), Hydreigon’s newly received exclusive move, which turns Mega Hydreigon into a scary fast hard-hitting sweeper that outspeeds even Deoxys-Speed, allowing it to pretty much outspeed the entire metagame and deal serious hurt to it at the same time.
But without Flash Cannon, Mega Hydreigon has no reliable method to dent Fairy-types, except for using Fire Blast against Mega Mawile or Klefki. Besides, it does not get a utilizable move to hit Fighting types that are a serious threat to it. Perhaps one of the most obvious counters to this is Azumarill which resists all of Mega Hydreigon’s moves(except Flash Cannon which is only neutral), and can hit back even harder with a STAB super effective Play Rough. Perhaps with the introduction of this Mega we can see a higher usage of Azumarill on people’s teams.
Final verdict: Ubers