I copy and pasted this post.  Ill post below on the few megas I've used

Charizard X:

If a Charizard set sees play, it'll be this one. Charizard X hits like a truck. I've been running:

Adamant;
Dragon Dance
Flare Blitz
Dragon Claw
Earthquake

to some success. He hits pretty hard if he can get his dance up, and he hits hard enough if he can't. It's difficult to effectively wall him. For a while, I was running Aerial Ace and kept getting walled by Mawile, until I realized I'm a fucking idiot and put Earthquake on him. Very very good Pokemon. NEEDS a rapid spinner on the team though, and, being a dragon, really appreciates having a Chandelure with Shadow Tag. Definitely see this guy as a top contender for OU.

EDIT: Turns out I'm an idiot again! Chandelure DOES NOT get Shadow Tag, I checked mine and, sure enough, it has Infiltrator. Luckily, my friends were pretty sure it had shadow tag too, and never tried switching out. Probably best not to rely on your opponent not knowing something though, highly recommend not using him. Let this paragraph stand as a monument to my sin.

Charizard Y:

Not much to say about Charizard Y. With team preview, you pretty much know which Charizard you're going to see. Is it a sun team? Yes? Charizard Y. If not, Charizard X. I suppose someone could run a set on a regular goodstuffs team for the shock value, but generally speaking, I think I'd rather have the X. If it can switch into X, it can probably switch into Y too.

Lucario:

This guy is insane. Like... really really good. I've been running:

*Adamant:
Swords Dance
Close Combat
ExtremeSpeed
Bullet Punch *

and have swept many a team. I thought Skarmory would wall it, but it gets 2hko'ed by Close Combat. I'm not really sure what to do about him, he just kills everything. I got killed a few times by varios things.... focus sash into Flamethrower, Jolly Garchomp... but his moveset is so huge, it would be easy to run Ice Punch and a jolly nature instead, and stealth rock support takes care of the sash. I almost see him going to uubers tbh. Another top contender.

Gengar:

This guy is the one that everyone is so sure is going to Uubers. To be honest, I could see it going either way. I've been running:

Modest (should be Timid)
Shadow Ball
Sludge Bomb
Focus Blast (was Thunderbolt)
Destiny Bond

This guy wrecks most things when he comes out. I've said that about a couple Pokemon now. If you're wondering which one is best, it's usually the one that Mega-evolved first, since it has its mega speed applied while the other pokemon is mega evolving and only has its regular speed factored. These megas are frail.

Anyway, I was running this on a team with Salamence and having huge success. Lead with him, megaevolve and hit whatever switched in, switch to whatever, get out Salamence, begin to wreck, switch into Gengar to take out the fairy they counter with, then continue wrecking with Salamence... or some variation of that. Mawile gave me problems though, which is why I eventually picked Destiny Bond over Thunderbolt. MegaMawile is a force of nature. Honestly, if this thing doesn't go to uubers, it's going to be thanks to MegaMawile keeping it in check. It was nice taking it down with Destiny Bond though, and then continuing to wreak havoc with Salamence.

Bannette:

This guy didn't impress me. I feel like Sabeleye or even Klefki is a better option. He doesn't do anything that another pokemon couldn't do just as well. MegaMawile can Sucker Punch twice as hard as Bannette and has better typing to boot, and there are other prankster pokemon you could be using. Seems like a waste of a MegaEvo slot personally, but maybe I was just using him wrong?

Absol:

MegaEvolving in response to a status or setup move was pretty cute in the beginning, but it didn't take long for my opponents to wisen up. At first, I would try to mega evolve him as soon as I could, so I could switch him in and bounce moves back. It really sucked losing my critical hits from super luck, and that's when I realized the greatest thing about Mega Absol, no one would risk using a status move on him for fear that he might go mega. I swapped the mega evolution for a choice scarf, and was allowed to crit without fear of being set up on. It's a neat way to mess with your opponent. I also ran Life Orb Alakazam on that team, for Magic Guard, and my mega was actually Gengar, for trapping fairies that would kill Absol. Maybe I just wasn't using the mega right, but it felt pretty underwhelming. The psychological warefare of having 3 pokemon capable of megaevolving was the best part.

Blastoise:

The important thing about Blastoise is that he is, hands down, THE best Spinner in the game. You can't do shit about him.

I've been running

Modest:
Water Pulse
Dark Pulse
Aura Sphere
Rapid Spin

And it's hilarious watching them switch in a spinblocker only to get obliterated by Dark Pulse. Mega Gengar, Chandelure, Aegislash, doesn't matter. They all go down. He's the most reliable spinner I've ever used, ranking even above Forretress (my all time favorite spinner before this). You can't pursuit him, you can't shadow ball him, he tanks everything and then kills you right back! The addition of Dark Pulse as a TM move, and Aura Sphere, and Dragon Pulse as egg moves from Clauwitzer is his saving grace.

Have honestly considered dropping Aqua Pulse for Dragon Pulse to deal with Mega Ampharos and other dragons. Everyone expects Water Pulse, so you don't even really need it on there. The threat of it is enough to get the job done.

I'll edit this either later tonight or tomorrow with Mega Manectric, Houndoom, Mawile, Pinsir, Gyarados, Medicham, and Ampharos.

EDIT: Can't stop won't stop. Here's my experience with Ampharos.

Ampharos

At first I thought I would make him a bulky special attacker. This was a mistake, sort of. I was using him wrong. He's supposed to be a volt switcher. Being able to volt switch through lightningrods and Volt Absorbs is pretty cool (though he's still walled by ground types). There wasn't shit I could do to Mawile, but Azumarill and Togekiss took some serious electric damage when they thought they could beat me. The lowered speed actually helps a little bit, since you can volt switch into your pokmeon without worrying about it taking damage. I liked switching into Dugtrio and Chandelure, because there's nothing like the look on your opponents face when thy know they're going to die and there's nothing they can do about it. All the Mega Gengar fans will probably try a similar team out if he goes uuber.

EDIT: Reminder, Chandelure DOES NOT get Shadow Tag. I'm just dumb, and lucky.

Medicham

I feel like this guy is a lot better than I gave him credit for when I tried him out. His attack gets fucking HUGE. Problme is, he's already holding an item. You can't exactly Scarf or Berry him. He's got great coverage between Hi Jump Kick, the punches, and Psycho Cut. I'm mentioning him here because I think someone else will break him. Either a Speed Boost passed onto him, or maybe a subsititue variant, or something... I didn't try any of that. I only thought of all this stuff after trying the other megas, and plan on going back and (hopefully) rediscovering a use for him.

Gyarados

I tried a Dragon Dance + 3 attacks moveset and failed miserably. Gengar could Focus Blast him, any fast electric type could kill him... it was pretty bad. But I was doing it wrong. One of my friends has had much more success running Waterfall, Dragon Dance, Rest, and Sleep Talk. With Mold Breaker, Waterfall can hit all the things! It seems pretty gimmicky, but the way he's got the EVs, taking Gyarados out is really difficult. My Mega Gengar could not do so through conventional means, and eventually resorted to destiny bond. It might be pretty good, but then again, it might just be a silly gimmick. Either way, Gyarados will be back as a bulky water, and the potential for a mega evolution will be there.

Pinsir

I love this guy, he is a BOMB. I actually mentioned him in an earlier comment.

Jolly; Hyper Cutter into Aeriolate
@Pinsirite
Swords Dance
Return
Quick Attack
Earthquake

You can't get Double Edge on him right now, it's a gen 3 only tutor move. Close Combat is worthless because Skarmory with Brave Bird can tank it and OHKO back with Brave Bird. Aside from Skarmory and a few Electric Fliers, Pinsir kills most things. It's pretty scary. I picked Hyper Cutter because Mega Evolution happens AFTER they switch, and when I had Moxie, it wasn't uncommon for people to switch into Mawile for the intimidate, neutering my poor Pinsir. It's not like regular Pinsir and his sorry 85 base speed could take advantage of moxie anyway. Earthquake is there for Aegislash.

Mawile

This guy is another one of my favorite megas. Sucker Punch fired off of a massive attack stat is hilarious. Gengars go down fast. Another beauty is the new fairy move, Play Rough. It's a pain getting Mawile to level 60, but so so worth it. The attack stat is stolen and then doubled with Huge Power. Mawile is bulky enough that it can stick around, doesn't just fall over and die to priority moves. This will probably be one of the most used Megas. I've been running a 252HP/252Atk Sucker Punch/Play Rough/Pain Split/Sword Dance set, but there's a lot of room for customization.

Houndoom

I ran a relatively simply Sunny day/Dark Pulse/Solarbeam/Flamethrower set. It was pretty good. Not amazing, not bad, but pretty good. It felt gimmicky, and easily countered. I would rather be playing Charizard Y for the drought/Solarbeam shenanigans as my mega on a sun team. Tried him out as a standalone outside of sun teams. He just feels really subpar, and I hate that, because he's one of my favorite mons.

Manectric

I haven't gotten a chance to really test this guy out yet. I'm still trying for an Electrike with HP Ice. Since Hidden Power got nerfed, not sure if it'll be as good, but we'll see. It looks like a better Jolteon, but I'm not sure if it's worth he megaslot.

Khangaskan

This mega was a joy to use. I don't know how competitive it will be, but the trick with Power Up Punch was really cute. I also ran Earthquake, Return, and Sucker Punch off a jolly nature. I got killed a lot by Ghosts and fast Fighting types. Didn't matter, it was really fun to lose with. I'm pretty sure it gets Drain Punch, so I might revisit it when I have time with a bulkier set. Problem is, it only has 4 moveslots and it really wants 5. If you go with a boosting move, like sword dance or power up punch, you lose coverage. If you don't, you lose power and have to re-allocate your EVs to a streamlined offensive sweeper. It's so close to being good... Maybe as a Baton Pass recipient? If you don't have a Medicham or Absol on hand I guess.

Will maybe edit later with the rest of the megas, I've come this far...

EDIT:

Alakazam/Scizor/Blaziken

These three go together, because all three do what the regular versions do. There's not really much reason to run these guys as megas. Life Orb Alakazam with Magic Guard hits about as hard as Mega Alakazam, but doesn't worry about entry hazards, weather, or poison/burn. Scizor gets some beefy defenses. I tried a bulky swords dance set. It was OK. Rally underwhelming. Life Orb Blaziken takes some damage from Life Orb, but either sweeps or doesn't sweep. The damage from Life Orb is negligible, because either you take out most of their pokemon or you get OHKOed.

Don't use these megas. Except Scizor, in Uubers maybe.

Abomasnow

Weather got nerfed. Are you running Hail? Then I guess you can use him, it's not like there are that many other megas you'd use in Hail.

Tyranitar

Alright, I may have overlooked this guy. He's actually really cool, he can extend your sandstorm past turn 5 by Megaevolving. I haven't gotten a chance to test him yet, I would need access to the rest of the sand team which I don't have right now. I don't really see him doing too well without the support of a dedicated sandstorm team, which is news to no one. I feel like the skill ceiling for new sandstorm teams will be higher and yield less of a reward.

This Mega gets massive points for looking like Tengen Toppa Gurren Lagann.

Mewtwo

I don't play Uubers as much as I play OU, so I feel like I can't accurately judge them. I will say that from what I've seen of Xerneas, MegaScizor seems like the play.

Garchomp

Slower Garchomp. If this guy has a use, I haven't found it.

/u/Strategolegends suggests Wallbreaker. I'm not really sure what a wallbreaker set though.

If I had to guess at a wallbreaker set though, I suppsoe I'd go with...

252Atk, 252 HP, def sp atk Fire Blast Earthquake Surf Iron Head.

I don't see much point in going Dragon Claw, and Outrage is just begging to see a fairy switch in. Fire Blast 2HKOs Skarmory, Surf 2HKO's Landurus. Earthuake 2HKOs Hippowdon, Iron Head 3HKOs Cresselia (unless it becomes fairy, then it 2HKOs). That's a start at least.

Aerodactyl

I don't know why you would ever want your suicide lead to go mega. If he isn't your suicide lead, then you could probably use a better physical sweeper, like Mega Lucario.

Venusaur

I love Venusaur so much, I really wanted to make this guy work. I put him on Chlorophyll Venusaur and attempted to sweep, then megaevolved and got in one massive hit at Chlorophyll Venusaurs mach speed when the sun was about to end. Then he continued his sweep as a tank. It was better than MegaHoundoom, but I think I'd have preferred MegaCharizard Y.

Aggron

He's one hell of a wall. I can't really figure out why you'd waste your mega on a wall though when Skarmory already does everything he does so well... There wasn't a moment I had him out that I didn't think "How is this better than Skarmory?"

Gardevoir

I honestly haven't gotten a chance to use her yet. She doesn't look like a better option than MegaMawile though, so I'm not very optimistic.

Heracross

I haven't even looked at him yet.

From my experience

Mega Absol -  While it is good at at least doing damage it is still frail and pretty much gets OHKO every time.

Mega Mawile - I don't see why people are making it look like it is amazing because with such a low speed and fire type being so common it gets OHKO a lot as well.  You will be relying on Sucker Punch a lot and hope for the best.  Still it is better than Mega Absol unless you somehow manage to do a Sword Dance with Mega Absol.

Mega Charizard X - Its boosted stats go to the wrong place.  Charizard is a special attacker but instead it gets stats on Attack.  It does benifit with a good typing though Dragon/Fire. Even then it isn't fast enough and can be taken out relatively easy.

Those are the only ones ive used and Im about tired of going against Blaziken in every match.  Thing is just stupidly OP.  Mega Absol may actually be better than Im giving him credit for but I have to deal with Blaziken every single time

The only thing holding the Mega Charizards back is they're base 100 speed. They could have focused on making Charizard X more physical and relegating the points they put into SpA into speed and attack. Same can be said in reverse for Mega Charizard Y. That being said, Mega Charizard X has phenominal typing and can tear it up. Tough Claws with base 130 attack combined with Dragon Dance... enough said.

Please... Somebody tell me... Did anybody try to breed Megahorn onto Pinsir yet? They finally gave flareon Flare Blitz.... I've waited so many years for Megahorn Pinsir.... It would take it up to another level.

Mega Gengar will kill your father, rape your mother, burn the house down, collect the insurance money AND get away scott free.

I REALLY want to try Mega Ampharos. Still working on it...

Aggron loses its Rock Type Weaknesses which is a blessing.... but also loses the stab that came with Headsmash w/o recoil. Nobody will take it on physically with that monstrous defense. It's special defense is only OK at base 80. Why use Aggron as Skarmory when Skarmory dose Skarmory better? Aggron can wall but why use it on a mega? It has great attack but it doesn't persuade me to use it over another more capable mega.

Mega Houndoom is a disappointment.... Its base stats are nearly identical to chandelure with only significantly higher speed and a marginally higher base HP. It's easily outclassed by other Megas that do a better job than it. THAT BEING SAID. Houndoom looks beastly and is one of my alltime favs. So glad they gave him a Mega.

And they still haven't released Shadow Tag Chandelure or what?

Blaziken was already uber, so giving him a mega seemed like a waste to me. Not even remotely surprised everyone is spamming it online.

Honestly it is kind of ruining online for me.  Im trying to be respectful and not use Blaziken because it is stupidly over-powered but it is so annoying people using him in almost every match. 

what build are they using? Mixed, special, or physical

looks physical. Shield one time and then it pretty much is faster than most things because the speed ability.  Though right now my Rotom-W is countering it.

Well Nintendo is embracing dlc and patches, so you never what could happen to this gens pokemon meta. Have GF ever confirmed that they follow it? Surely they do.