I started this post before I saw Langur's, so forget what I said. I will provide this one set, though:
Mega Aerodactyl v2: 80/135/85/70/95/150 (Hey, that's exactly the same as the regular mega!)
Aerodactyl @ Mega Stone
Ability: Serene Grace
EVs: 4 HP / 252 Atk / 252 Spe
Jolly Nature
- Protect
- Rock Slide
- Bite
- Iron Head / Roost
Protect is mandatory to reach the speed tier required to outpace base 130-149 and tie with other 150s, as well as Scarfers with less than base 84 Speed. The point of the set is to use 60% flinching (54% with Rock Slide if you factor in the miss rate) to chip away at teams that lack priority (which, unfortunately, are very few). Bite is there to hit Steel-types neutrally, making the weakening process slightly quicker (+4 BP and +10% accuracy over NVE STAB Rock Slide). Iron Head is an option for Fairies and for raw power, while Roost is an option in case recovery is needed.
Also, I'll just put this here:
Dewgong: 90/100/100/80/115/90
Cloyster: 50/125/200/95/65/90 (irrelevant; just use Skill Link)
Lapras: 130/115/100/95/115/80
Glalie: 80/110/100/90/100/100 (new mega) or 80/120/80/120/80/100 (old mega)
Abomasnow: 90/122/95/102/105/80
Weavile: 70/150/85/55/105/145
Mamoswine: 110/160/100/80/80/100
Froslass: 70/110/90/90/90/130
Beartic: 95/140/100/80/100/70
Kyurem: 125/160/110/140/110/115
Kyurem-B: 125/200/120/130/110/115
Avalugg: 95/147/204/54/66/48
Cloyster: 50/125/200/95/65/90 (irrelevant; just use Skill Link)
Lapras: 130/115/100/95/115/80
Glalie: 80/110/100/90/100/100 (new mega) or 80/120/80/120/80/100 (old mega)
Abomasnow: 90/122/95/102/105/80
Weavile: 70/150/85/55/105/145
Mamoswine: 110/160/100/80/80/100
Froslass: 70/110/90/90/90/130
Beartic: 95/140/100/80/100/70
Kyurem: 125/160/110/140/110/115
Kyurem-B: 125/200/120/130/110/115
Avalugg: 95/147/204/54/66/48
Jynx: 65/60/55/145/115/115
Lapras: 130/95/100/115/115/80
Glaceon: 65/70/130/160/115/85
Kyurem: 125/140/110/160/110/115
Aurorus: 123/87/92/129/112/78
Lapras: 130/95/100/115/115/80
Glaceon: 65/70/130/160/115/85
Kyurem: 125/140/110/160/110/115
Aurorus: 123/87/92/129/112/78
Pidgeot: 83/110/95/80/90/121 (new mega)
Fearow: 65/120/85/71/81/120
Farfetch'd (heh): 52/95/75/68/82/80
Dodrio: 60/140/90/70/80/120
Scyther: 70/140/100/65/100/125 (assuming NFEs can mega)
Pinsir: 65/155/120/65/90/105 (old mega only)
Gyarados: 95/155/99/70/120/101 (new mega only)
Aerodactyl: 80/135/85/70/95/150 (new mega only)
Dragonite: 91/164/115/110/120/100
Crobat: 85/120/100/80/100/150
Jumpluff: 75/85/90/65/115/130
Skarmory: 65/110/160/50/90/90
Swellow: 60/115/80/60/70/145
Ninjask: 61/120/65/60/70/180
Altaria: 75/100/110/80/125/100 (new mega only)
Salamence: 95/165/100/120/100/120 (new mega...if the old mega is banned, this one should be as well imo)
Staraptor: 85/150/90/60/80/120
Mothim: 70/124/70/104/70/86
Vespiquen: 70/110/122/90/122/60
Honchkrow: 100/155/72/115/72/91
Gliscor: 75/125/145/55/95/115
Unfezant: 80/145/100/75/75/113
Archeops: 75/170/85/122/85/130
Swanna: 75/117/83/97/83/118
Emolga: 55/105/80/85/80/123
Braviary: 100/153/95/67/95/100
Mandibuzz: 110/95/125/65/115/100
Landorus-I: 89/165/110/125/100/121
Landorus-T: 89/175/110/115/100/111
Talonflame: 78/111/91/84/89/146
Hawlucha: 78/122/95/84/83/138
Fearow: 65/120/85/71/81/120
Farfetch'd (heh): 52/95/75/68/82/80
Dodrio: 60/140/90/70/80/120
Scyther: 70/140/100/65/100/125 (assuming NFEs can mega)
Pinsir: 65/155/120/65/90/105 (old mega only)
Gyarados: 95/155/99/70/120/101 (new mega only)
Aerodactyl: 80/135/85/70/95/150 (new mega only)
Dragonite: 91/164/115/110/120/100
Crobat: 85/120/100/80/100/150
Jumpluff: 75/85/90/65/115/130
Skarmory: 65/110/160/50/90/90
Swellow: 60/115/80/60/70/145
Ninjask: 61/120/65/60/70/180
Altaria: 75/100/110/80/125/100 (new mega only)
Salamence: 95/165/100/120/100/120 (new mega...if the old mega is banned, this one should be as well imo)
Staraptor: 85/150/90/60/80/120
Mothim: 70/124/70/104/70/86
Vespiquen: 70/110/122/90/122/60
Honchkrow: 100/155/72/115/72/91
Gliscor: 75/125/145/55/95/115
Unfezant: 80/145/100/75/75/113
Archeops: 75/170/85/122/85/130
Swanna: 75/117/83/97/83/118
Emolga: 55/105/80/85/80/123
Braviary: 100/153/95/67/95/100
Mandibuzz: 110/95/125/65/115/100
Landorus-I: 89/165/110/125/100/121
Landorus-T: 89/175/110/115/100/111
Talonflame: 78/111/91/84/89/146
Hawlucha: 78/122/95/84/83/138
Noctowl: 100/60/70/106/116/90
Altaria: 75/80/110/100/125/100 (new mega only)
Chatot: 76/75/65/122/62/111
Togekiss: 85/60/115/150/135/100
Noivern: 85/80/100/127/100/143
Altaria: 75/80/110/100/125/100 (new mega only)
Chatot: 76/75/65/122/62/111
Togekiss: 85/60/115/150/135/100
Noivern: 85/80/100/127/100/143
Granbull: 90/150/95/70/80/65
Mawile: 50/115/105/65/75/70 (new mega; old mega is banned)
Altaria: 75/110/110/110/105/80 (old mega only)
Klefki: 57/110/111/90/107/95 (irrelevant; just use Prankster)
Diancie: 50/130/170/110/170/70
Mawile: 50/115/105/65/75/70 (new mega; old mega is banned)
Altaria: 75/110/110/110/105/80 (old mega only)
Klefki: 57/110/111/90/107/95 (irrelevant; just use Prankster)
Diancie: 50/130/170/110/170/70
Clefable: 95/80/93/125/110/80
Wigglytuff: 140/80/65/115/70/65
Azumarill: 100/60/100/90/100/70 (irrelevant; just use Huge Power)
Gardevoir: 68/75/85/155/135/100 (new mega) or 68/85/65/165/135/100 (old mega)
Altaria: 75/110/110/110/105/80 (old mega only)
Togekiss: 85/60/115/150/135/100
Sylveon: 95/75/85/140/150/80
Wigglytuff: 140/80/65/115/70/65
Azumarill: 100/60/100/90/100/70 (irrelevant; just use Huge Power)
Gardevoir: 68/75/85/155/135/100 (new mega) or 68/85/65/165/135/100 (old mega)
Altaria: 75/110/110/110/105/80 (old mega only)
Togekiss: 85/60/115/150/135/100
Sylveon: 95/75/85/140/150/80
Anyway, what this metagame brings, huh? Well, because there is only one mega per team and any Pokémon can Mega Evolve, there is an element of unpredictability; you (almost) never know which Pokémon is going to Mega Evolve until it's too late. It seems to encourage offense more, seeing how new megas get +30 in their favored offense, +10 in the other, and +20 in Speed, but the +20 in both defenses is not to be overlooked. It may not seem like much of a difference, but...
252+ Atk Adaptability Crawdaunt Crabhammer vs. 252 HP / 252+ Def Fur Coat Blissey: 242-285 (33.8 - 39.9%) -- guaranteed 3HKO
252+ Atk Adaptability Crawdaunt Crabhammer vs. 252 HP / 252+ Def Fur Coat Blissey: 181-213 (25.3 - 29.8%) -- guaranteed 4HKO
In this case (the Adaptability Mega Crawdaunt set vs. the Fur Coat Blissey set, 10 Def in the first calculation and 30 Def in the second calculation), it's the difference between being 3HKOd and being 4HKOd.
The abilities involved do not have any particular bias, some being offensive (e.g. Adaptability, Tough Claws, Sheer Force, the -ates, Gale Wings) and some being defensive (e.g. Fur Coat, Intimidate, Prankster, Magic Bounce, type-immunity abilities), so it seems pretty balanced.
Speaking of Intimidate, I'm assuming that if a Pokémon's base form has Intimidate and its Mega's ability is Intimidate, sending the Pokémon in will cause one Intimidate and Mega Evolving the Pokémon will cause another Intimidate. If that's so, then double Intimidate is a thing. You may be thinking, "Fur Coat is better, though, isn't it?" It is, but double Intimidate is more effective on Mold Breakers.
But again, since every team is allowed only one Mega, I feel the focus of discussion should not only be on the Megas, but also the regular Pokémon who may very well have to put up with the Megas. Mold Breaker is definitely worth considering for anti-stall (non-Mega users: Pinsir, Rampardos, Haxorus, Pangoro, Excadrill, Throh, Sawk, Basculin, Druddigon, Hawlucha), and the ones that can learn Stealth Rock (Pinsir, Rampardos, Excadrill, Druddigon) are considerable options in the lead position as a measure against Magic Bounce Megas.
Meanwhile, Pokémon with Defiant (Bisharp, Primeape, lolFarfetch'd, Purugly, Braviary, Tornadus-I, Thundurus-I) and Competitive (Wigglytuff, Milotic, Gothitelle, Meowstic-F) are suitable options for dealing with the "double Intimidate" process described earlier, although most of them are nothing short of situational and special attackers have literally no problem with Intimidate.
Shedinja is another notable option as it can block any form of Rapid Spin except for Aerilate, which only Delibird (heh) gets STAB on and that only Fighting-types (read: the Hitmons) would use for coverage. (Just so it's obvious, I don't mean using Rapid Spin as an attacking move, I mean running Return alongside it.) In other words, the Excadrill set that The Reptile suggested earlier? It can only hit Shedinja in its base form with Mold Breaker Earthquake. It's also immune to PixiSpin, although I don't see that being nearly as common. However, even though Shedinja is immune to many common types, it can be easily dispatched through super-effective hits (read: birdspam), Mold Breaker, hazards, status, and/or weather.
Because setup and Contrary will also be quite prominent, an Unaware user can come in handy in times of dire need. It is more advisable to use a Mega for this role if anything, mainly because of the Victini set that FlameUser64 provided. Clefable, no matter what form of defensive it is, gets 2HKOd by V-create and/or Overheat and can't do anything back, while Quagsire needs 60 Attack EVs to guarantee a 2HKO with Earthquake given neutral nature and, with 252/196+, is most likely (98.6% chance) 3HKOd by Focus Blast and potentially (87.5% chance) OHKOd by Grass Knot if Victini is running it. (For the record, the 3HKO and OHKO are guaranteed after Stealth Rock.) At any rate, the spread is not safe from the Adaptability Crawdaunt set; Crabhammer is a guaranteed 2HKO while Quagsire cannot even 2HKO back (also Crawdaunt is faster). Inquiry: What is the best choice of Unaware Mega?
Prankster in conjunction with Haze from Murkrow or, in the case of Megas, Topsy-Turvy on Malamar or Heart Swap on Manaphy, could also be used against setup. Be warned that Malamar has next to no other support options and Manaphy can't do much with acquired boosts save for Knock Off and/or Waterfall. Sadly, though, there is a problem with Prankster: it can't hold a candle to opposing priority due to the given inability to outspeed and the fact that Extreme Speed is a thing.
So, the interesting thing about this metagame is that there is no be-all-end-all team of six; there are many, many Pokémon that can potentially go Mega and either forge a sweep or halt a sweep, and some teams can be designed such that it's as difficult as possible to tell which Pokémon is going to Mega. I mean, you could be cheeky and use Frisk, but who would use a precious Pokémon/ability slot for that?
Phew. Words.