THIS THREAD IS A WORK IN PROGRESS
Mega-evolution in doubles is vastly different to singles, much like the battles themselves. What is viable in doubles which, while for the most part is similar to singles, can be entirely different just due to the sheer difference between the length of battles and how we battle.
The Fallen
Mega-gengar has suffered many blows in his transition between singles and doubles. No doubt his best set in the doubles metagame right now is Perish Song, as he can still trap and stall somewhat, removing two of your Pokemon for little cost. The problem, of course, is that with some doubles battles being 5-6 turns Perish Song takes far too long to take it's full effect (I learned this myself trying out a Perish Song Honchkrow) and by the time Perish Song activates Mega-Gengar has already been killed and you've switched out. Looking at other possible sets Mega-Gengar could run... nothing looking overly good. 170 SpAtk is fantabulous, but the 130 speed is actually a killer against the common trick room teams, and having no reliable spread move really hurts his viability for offensive sets. Mega-Gengar is popular as a Pokemon but soon people will realize that Mega-Gengar really cannot stand up to his standards in singles in the doubles environment.
Mega-Venusaur is actually rather popular in the XY singles metagame as a pretty great tank and looking at his stats it is easy to see why. 80/123/120 defenses are pretty fantastic and 100/122 offenses aren't too shabby either. It has a fairly good 80 base speed for doubles which lets it either check Trick Room teams or be speedy with support moves such as Tailwind. The chlorophyll ability also helps Mega-Venusaur take Heat Waves and Blizzards aimed at your team quite well, but while defensively Mega-Venusaur looks pretty up-to-scratch Grass/Poison is not a good offensive typing and actually, it's movepool isn't very good offensively either. Now, the typing is rather good defensively but let's have a look at it's movepool... it has... Light Screen I guess? Nah, Mega-Venusaur has some pretty good utility in actual fact - Safeguard, Light Screen, Aromatherapy, Knock Off and Sleep Powder really stand out to me as the best moves he has in his arsenal, and unfortunately the majority of other Grass-Type Pokemon can do this. Mega-Venusaur will struggle to find use on teams in the doubles metagame but I have no doubt in my mind he will find a niche in due course, just don't expect it to be as good as it is in singles.
The just-as-good-here-as-in-singles Pokemon
Honestly Mega-Gardevoir can be a serious threat, no joke. Dazzling Gleam, like I mentioned with Mega-Mawile, is a spread move and coming off of that 165 Special Attack stat is just fantastic. Her speed is 100 meaning that backed by Tailwind or really without she can be a seriously fast powerful Pokemon and her typing is actually pretty great making her.. well..not weak to any common moves in doubles. Like at all. The danger still lies in her ok-ish bulk, 68/65/135 is not terrible in terms of Special Defense but looking at physical defense that is scarily low for the doubles environment where Earthquakes and Rock Slide are absolutely everywhere Mega-Gardevoir is rather heavily reliant on team matchup. She is a threat to behold against several types of sun teams and can be a rather great thing to have against hail/blizzspam due to her high Special Defense and high Special Attack. Her typing gives her neutral coverage on a bunch of the tier, in fact I think only Dark and Steel resist both of her STABs and guess what? Focus Blast destroys them! This means that in doubles Mega-Gardevoir has the power to demolish a good majority of the tier in some way or another and has room to spare for Protect without having to give up on a type, this would make her quite suited to doubles however defensively her typing doesn't do her any favours. She's resistant to Fighting, Psychic and immune to Dragon but only Fighting is common out of those types, whereas she's weak to Ghost, Poison and Steel which are oddly common lately. She arguably took a dip from singles but with the right support Mega-Gardevoir is actually a pretty fantastic threat and something pretty much every team needs to prepare for.
What can I say about Mega-Kangaskhan that hasn't been said already? He's powerful, has perfect neutral coverage between two moves (practically, I guess Ghost Types with Levitate betray this rule but now that most Gengar are Mega now it's not as significant) and he finds use in doubles as well. All you need to do to make Mega-Kagnaskhan work in doubles is slap an Ammoonguss as a partner an get it to use Rage Powder a bunch of times while Kangaskhan proceeds to destroy the other team. You can also simply use Earthquake while your partner uses Protect and have the other team get at least one of their Pokemon to faint. Return is by far the thing that you want to use on Mega-Kangaskhan the most and really you can just use the singles set minus Power Up Punch plus Protect and proceed to wreak havoc. Honestly the most straight-forward and easiest Mega to use.
Mega-Alakazam can be rather good in doubles even though neither his stats nor movepool would seem to suggest so. It does get Dazzling Gleam for an un-STAB'd spread attack but most of the time you want to focus on removing the opponent's team one Pokemon at a time with Psychic or Focus Blast. I said for Mega-Kangaskhan that he is probably the most basic Mega there is but Mega-Alakazam gives him a run for his money in this respect. Was Alakazam really difficult last generation? Well that thing at least had Magic Guard, Mega-Alakazam is even more basic!
Mega-Mawile is a monster in singles and in doubles she is a monster too! She has one of the best typings in the game, only weak to Fire and Ground. However, both Heat Wave and and Earthquake are hilariously common in doubles, Mega-Mawile greatly appreciates Wide Guard support from the likes of
Vast improvement from singles to doubles
Mega-Charizard-Y, or for the purpose of this ChariYard, is absolutely incredible in the doubles format and arguably the best Mega in both this category and in general for doubles. The immediate sun lets it support it's team by switching in but, unlike Ninetails or (lol)Vulpix, the guy's not a deadweight, as in, he can do something! ChariYard has Heat Wave and SolarBeam and that's all he needs. The only issue I have with MegaYard is that he can't be paired with Mega-Houndoom since these two as Heat Wave spammers would completely demolish teams and teams alike ^^. ChariYard does kinda suffer from the Drought nerf in Singles, in Doubles this thing actually doesn't need more than those 5 turns most of the time. It is entirely dependent on the sun though so it does run a small risk of being stalled out but it is quite difficult to do so. ChariYard is going to be one of the up-most strongest power-houses of smogon doubles there is no doubt in my mind of this. Long live the king!
Mega-Scizor has some of the greatest things that you need on a good doubles Pokemon. Powerful priority, hard-hitting attacks outside of priority and support moves - all of which can fit on one set. To show you what I mean on my set I've been running Bullet Punch - priority - Bug Bite - hard hitting otherwise - and Light Screen - support. And obviously Protect. Strangely enough Mega-Scizor also has access to a couple of flying-type support moves as well in Defog and Tailwind. Defog sounds odd, but actually it can support moves such as Blizzard and let them destroy whereas Tailwind is fantastic to fit on
anything. This makes Mega-Scizor incredibly diverse and a dangerous threat. Unfortunately, Mega-Scizor is weak to the common Heat Wave and Mega-Charizard-Y but pick your fights and Protect against the rest of them and Mega-Scizor could be one of the most dangerous Pokemon on your team.
Loreal Sheep, or Mega-Ampharos, is terrible in singles due to her sheer pathetic speed, but here in doubles we say if you're slow all the power to you, we'll stick you with a Jellicent and you can wreck everything with powerful Thunders! This is exactly what I mean about Mega-Amphy, the thing is strong. Not as strong and Mega-Gengar or Mega-Alakazam but they are far frailer and get destroyed by Trick Room. Mega-Ampharos? She relishes it! Honestly the amount of megas who can really abuse Trick Room is disappointingly small, and by small I mean we have Mega-Ampharos, Mega-Mawile who sucks even in Trick Room, Mega-Aggron who again, loses Rock Head and only get STAB on Steel - a.k.a bad, and Mega-Abomasnow who wants to fill an entirely different niche. If you run Trick Room, run Mega-Ampharos, use this for 80% of Trick Room teams in the meta and it will be healthy. Discharge is another fantastic option for Loreal Sheep, if you can afford to use Protect on your partner Pokemon every other turn, of course. The main thing that I don't like is actually Mega-Ampharos doesn't get Draco Meteor. So many times I would have loved to use Draco Meteor with this thing but.. tutor move in Pokemon Z? :D
Any viable megas I haven't mentioned? Post them here! Post your thoughts on the ones I have posted about and if you think my summary is accurate! :)
Small e: Just seen Mawile is S-Rank in the viability ranks I guess that's a good place to start discussion since I don't see why she deserves a slot up there lol.
Mega-evolution in doubles is vastly different to singles, much like the battles themselves. What is viable in doubles which, while for the most part is similar to singles, can be entirely different just due to the sheer difference between the length of battles and how we battle.
The Fallen

Mega-gengar has suffered many blows in his transition between singles and doubles. No doubt his best set in the doubles metagame right now is Perish Song, as he can still trap and stall somewhat, removing two of your Pokemon for little cost. The problem, of course, is that with some doubles battles being 5-6 turns Perish Song takes far too long to take it's full effect (I learned this myself trying out a Perish Song Honchkrow) and by the time Perish Song activates Mega-Gengar has already been killed and you've switched out. Looking at other possible sets Mega-Gengar could run... nothing looking overly good. 170 SpAtk is fantabulous, but the 130 speed is actually a killer against the common trick room teams, and having no reliable spread move really hurts his viability for offensive sets. Mega-Gengar is popular as a Pokemon but soon people will realize that Mega-Gengar really cannot stand up to his standards in singles in the doubles environment.

Mega-Venusaur is actually rather popular in the XY singles metagame as a pretty great tank and looking at his stats it is easy to see why. 80/123/120 defenses are pretty fantastic and 100/122 offenses aren't too shabby either. It has a fairly good 80 base speed for doubles which lets it either check Trick Room teams or be speedy with support moves such as Tailwind. The chlorophyll ability also helps Mega-Venusaur take Heat Waves and Blizzards aimed at your team quite well, but while defensively Mega-Venusaur looks pretty up-to-scratch Grass/Poison is not a good offensive typing and actually, it's movepool isn't very good offensively either. Now, the typing is rather good defensively but let's have a look at it's movepool... it has... Light Screen I guess? Nah, Mega-Venusaur has some pretty good utility in actual fact - Safeguard, Light Screen, Aromatherapy, Knock Off and Sleep Powder really stand out to me as the best moves he has in his arsenal, and unfortunately the majority of other Grass-Type Pokemon can do this. Mega-Venusaur will struggle to find use on teams in the doubles metagame but I have no doubt in my mind he will find a niche in due course, just don't expect it to be as good as it is in singles.
The just-as-good-here-as-in-singles Pokemon

Honestly Mega-Gardevoir can be a serious threat, no joke. Dazzling Gleam, like I mentioned with Mega-Mawile, is a spread move and coming off of that 165 Special Attack stat is just fantastic. Her speed is 100 meaning that backed by Tailwind or really without she can be a seriously fast powerful Pokemon and her typing is actually pretty great making her.. well..not weak to any common moves in doubles. Like at all. The danger still lies in her ok-ish bulk, 68/65/135 is not terrible in terms of Special Defense but looking at physical defense that is scarily low for the doubles environment where Earthquakes and Rock Slide are absolutely everywhere Mega-Gardevoir is rather heavily reliant on team matchup. She is a threat to behold against several types of sun teams and can be a rather great thing to have against hail/blizzspam due to her high Special Defense and high Special Attack. Her typing gives her neutral coverage on a bunch of the tier, in fact I think only Dark and Steel resist both of her STABs and guess what? Focus Blast destroys them! This means that in doubles Mega-Gardevoir has the power to demolish a good majority of the tier in some way or another and has room to spare for Protect without having to give up on a type, this would make her quite suited to doubles however defensively her typing doesn't do her any favours. She's resistant to Fighting, Psychic and immune to Dragon but only Fighting is common out of those types, whereas she's weak to Ghost, Poison and Steel which are oddly common lately. She arguably took a dip from singles but with the right support Mega-Gardevoir is actually a pretty fantastic threat and something pretty much every team needs to prepare for.

What can I say about Mega-Kangaskhan that hasn't been said already? He's powerful, has perfect neutral coverage between two moves (practically, I guess Ghost Types with Levitate betray this rule but now that most Gengar are Mega now it's not as significant) and he finds use in doubles as well. All you need to do to make Mega-Kagnaskhan work in doubles is slap an Ammoonguss as a partner an get it to use Rage Powder a bunch of times while Kangaskhan proceeds to destroy the other team. You can also simply use Earthquake while your partner uses Protect and have the other team get at least one of their Pokemon to faint. Return is by far the thing that you want to use on Mega-Kangaskhan the most and really you can just use the singles set minus Power Up Punch plus Protect and proceed to wreak havoc. Honestly the most straight-forward and easiest Mega to use.

Mega-Alakazam can be rather good in doubles even though neither his stats nor movepool would seem to suggest so. It does get Dazzling Gleam for an un-STAB'd spread attack but most of the time you want to focus on removing the opponent's team one Pokemon at a time with Psychic or Focus Blast. I said for Mega-Kangaskhan that he is probably the most basic Mega there is but Mega-Alakazam gives him a run for his money in this respect. Was Alakazam really difficult last generation? Well that thing at least had Magic Guard, Mega-Alakazam is even more basic!

Mega-Mawile is a monster in singles and in doubles she is a monster too! She has one of the best typings in the game, only weak to Fire and Ground. However, both Heat Wave and and Earthquake are hilariously common in doubles, Mega-Mawile greatly appreciates Wide Guard support from the likes of
Vast improvement from singles to doubles

Mega-Charizard-Y, or for the purpose of this ChariYard, is absolutely incredible in the doubles format and arguably the best Mega in both this category and in general for doubles. The immediate sun lets it support it's team by switching in but, unlike Ninetails or (lol)Vulpix, the guy's not a deadweight, as in, he can do something! ChariYard has Heat Wave and SolarBeam and that's all he needs. The only issue I have with MegaYard is that he can't be paired with Mega-Houndoom since these two as Heat Wave spammers would completely demolish teams and teams alike ^^. ChariYard does kinda suffer from the Drought nerf in Singles, in Doubles this thing actually doesn't need more than those 5 turns most of the time. It is entirely dependent on the sun though so it does run a small risk of being stalled out but it is quite difficult to do so. ChariYard is going to be one of the up-most strongest power-houses of smogon doubles there is no doubt in my mind of this. Long live the king!

Mega-Scizor has some of the greatest things that you need on a good doubles Pokemon. Powerful priority, hard-hitting attacks outside of priority and support moves - all of which can fit on one set. To show you what I mean on my set I've been running Bullet Punch - priority - Bug Bite - hard hitting otherwise - and Light Screen - support. And obviously Protect. Strangely enough Mega-Scizor also has access to a couple of flying-type support moves as well in Defog and Tailwind. Defog sounds odd, but actually it can support moves such as Blizzard and let them destroy whereas Tailwind is fantastic to fit on
anything. This makes Mega-Scizor incredibly diverse and a dangerous threat. Unfortunately, Mega-Scizor is weak to the common Heat Wave and Mega-Charizard-Y but pick your fights and Protect against the rest of them and Mega-Scizor could be one of the most dangerous Pokemon on your team.

Loreal Sheep, or Mega-Ampharos, is terrible in singles due to her sheer pathetic speed, but here in doubles we say if you're slow all the power to you, we'll stick you with a Jellicent and you can wreck everything with powerful Thunders! This is exactly what I mean about Mega-Amphy, the thing is strong. Not as strong and Mega-Gengar or Mega-Alakazam but they are far frailer and get destroyed by Trick Room. Mega-Ampharos? She relishes it! Honestly the amount of megas who can really abuse Trick Room is disappointingly small, and by small I mean we have Mega-Ampharos, Mega-Mawile who sucks even in Trick Room, Mega-Aggron who again, loses Rock Head and only get STAB on Steel - a.k.a bad, and Mega-Abomasnow who wants to fill an entirely different niche. If you run Trick Room, run Mega-Ampharos, use this for 80% of Trick Room teams in the meta and it will be healthy. Discharge is another fantastic option for Loreal Sheep, if you can afford to use Protect on your partner Pokemon every other turn, of course. The main thing that I don't like is actually Mega-Ampharos doesn't get Draco Meteor. So many times I would have loved to use Draco Meteor with this thing but.. tutor move in Pokemon Z? :D
Any viable megas I haven't mentioned? Post them here! Post your thoughts on the ones I have posted about and if you think my summary is accurate! :)
Small e: Just seen Mawile is S-Rank in the viability ranks I guess that's a good place to start discussion since I don't see why she deserves a slot up there lol.
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