Miltank (AND Hitmontop) gets Scrappy Spin as well. Plus, Flygon gets flipping CONTRARY. CONTRARY.
I'm going to go over every "broken" 'mon you went over.
Gengar: Priority kills it, and we have many more Unawares. Besides, the pink blobs wall it hard, even with a boost AND Focus Blast. It's forced to run Focus Sash due to its frailty, and almost anything that it can't force out can OHKO it after a Smash.
M-Gardevoir: My experiences in the metagame are kinda weird, but most Garde I've seen haven't been offensive QD, but... physically bulky QD? Yeah, physically defensive QD MGarde is very anti-meta and takes a surprising amount of hits due to Intimidate pre-mega.
M-Metagross: Not that powerful, easily wallable if you choose the wrong coverage, Superpower screws your setup over. If you use Grass Knot for Quagsire, you have only two attacking moves left. Crazy 4MSS on this thing.
Garchomp: It's the Adaptability you should be worried about. Still, both STABs have immunities and powerful priority is abundant. Probably suspect-worthy, though.
Victini: Believe me, it was only broken due to Contrary + Overheat/Leaf Storm/Draco/V-create.
Hydreigon: Mach Punch kills it, as does Ice Shard and strong enough neutral priority. Believe me, priority is not only important in this metagame, but abundant as well. Scarf Meloetta-Pirouette (which will be very common, due to its great stats) outspeeds and OHKOs with Close Combat. Still walled by Azumarill. Priority is abundant.
Volcarona: Stealth Rock. The newly buffed Talonflame (gets EQ and Superpower). Mega Pinsir (Fake Out + Quick Attack, anyone?) Powerful priority is abundant. If it runs QD + Roost, it has huge 4MSS.
Landorus-I: This might be a problem. Banned it in 2.0, and it will likely be suspected in 3.0. Still, powerful priority is abundant.
Kyurem-N and Kyurem-B: Might be problems as well. Likely suspects in 3.0.
Diancie-M: Has to choose between physical STAB, special STAB, or going mixed. Stuff like physically defensive Chesnaught walls fully physical DD variants.
Maybe not gengar considering it has a hard time setting up, but what are the blobs going to do it to? Gengar can set up in their faces and ohko them.
M-Gardevoir is almost certainly better as an offensive mon than a bulky mon. After a quiver dance it outspeeds almost everything in the game and can ohko everything except victini, jirachi, bronzong, metagross, and chansey. And all of them get 2hko'd.
M-Metagross: "Not that powerful" Are you joking? 145 base attack plus the equivilant of a life orb on top? Quagsire is only 1 pokemon, although I am less convinced that it is broken looking at the amount of walls that get will-o-wisp now.
Garchomp: I forgot that it gets adaptability from mega lucario, that makes it even more broken lol. Now almost every wall in the game gets ohko'd bar skarmory and bronzong, and everything faster than it bar the few fast rock types/gengar still get ohko'd.
Victini: It gets swords dance, extremespeed and leaf blade to go with v-create. Probably the only counter is heatran, but that gets destroyed if the opponent chooses to use bolt strike. It could even just go choice band if you want.
Hydreigon and volcarona: lol, I know powerful priority is common in this meta. But that isn't always an answer:
252+ Atk Conkeldurr Mach Punch vs. 0 HP / 0 Def Hydreigon: 168-198 (51.6 - 60.9%) -- guaranteed 2HKO
Kyurem: Stronger than landorus: rock polish, ice beam, earth power, secret sword kills everything, also has enough bulk to live priority like hydreigon.
Kyurem-B: Gets close combat, which would be enough imo, but it also can run the same set as kyurem, except with a stronger fighting move and bolt strike if it wants. there are like 3 broken sets, needs to get out of here
Diancie gets dragon dance with earthquake+diamond storm coverage, along with extremespeed. It also gets nasty plot, so it could run a set with moonblast, earth power and thunderbolt for perfect coverage.