Hello everybody, I've finally decided to get around to this: the ADV DOU Viability Rankings! (at least my own opinion, I'm sure
xzern has some different opinions) Without further ado, let us begin!
S Rank: Metagross.

Metagross
The undisputed #1 of the tier. It's almost throwing if you aren't using this Pokemon. You REALLY have to justify not using it. Explosion can get you out of some really bad situations, Meteor Mash hits like a truck, and its ability to sponge hits is awesome. It isn't without flaw, and one of the bigger issues is wanting 5 move slots. Having to explode on Zapdos because your 4th move is Earthquake is pretty annoying. And while it can sponge a few hits, it is pretty susceptible to being worn down by multiple strong attacks, like Zapdos Thunderbolts. It is forced to explode when faced by Water types. Really susceptible to burns without a Lum Berry, and not using Leftovers means that your longevity is greatly reduced.
A+ Rank: Pokemon that can fit on a large amount of teams and will consistently contribute.

Gengar
Incredibly splashable. Support sets are annoying with Taunt and Wisp, and offensive sets put up a ton of pressure with Thunderbolts and Ice Punches. It can grab some sneaky kills with Explosion, while also being immune to Explosion. Earthquake immunity is pretty nice as well. It is very frail, and can be easily dispatched with a hit or two. It hates Deoxys-Speed's existence a lot, as Deo-S way outspeeds it and shuts down anything it may try to pull. Metagross + Gengar is a crazy good lead combination, so much so that myself and others are beginning to counterlead.

Zapdos
Thunderbolt is just as spammable in Doubles as it is in Singles, and Thunder Wave provides a crazy good amount of team support. It isn't nearly as reliant on Hidden Power in Doubles, as it has a teammate to cover the Pokemon it can't hit. Rock Slide being cut by 50% is crazy good for Zapdos. It can reliably sponge a Tyranitar Rock Slide, and non-STAB Rock Slides bounce off of it. It has a surprising amount of bulk, and can live a lot more than you might expect. It hates the omnipresence of Ice moves in the tier, from Deo-S, Gengar, and the plethora of Water types. It also hates being slowed, either through status or through moves like Icy Wind.
A Rank: Pokemon that are splashable, but have flaws that prevent them from being amazing.

Deoxys-Speed
An abundance of Speed is a massive boon for this Pokemon, as it coincides with some great coverage. Fast Taunt can be pretty nasty against weather teams. Despite having some excellent coverage, it really doesn't pack much of a punch. Moves like Fire Punch tend to bounce off of targets like Metagross, and it only really does significant damage if the target is frail or 4x weak. For this reason, it's very easy to send this thing out early and have it do nothing because the opponent isn't sufficiently chipped for Deo-S kills. Ice Beam still hits everything pretty hard, but your other moves really leave you wanting more. Its general frailty is also disappointing. It can eat one hit from just about anything, but don't expect more than that.
A- Rank: Pokemon that can fit on a large amount of teams, but have a few glaring weaknesses.

Tyranitar
This Pokemon is not nearly as splashable as in Singles, but still pretty good. The largest issue with Tyranitar is the 50% cut to Rock Slide, as it means your damage will be middling off of your STAB. You could opt to run something like Hidden Power Rock to slam Pokemon like Zapdos, but you're already strapped for move slots as is. Special/Mixed Tyranitar is still threatening, just less so with the existence of various forms of speed control and Water types.

Moltres
A very solid answer to the best Pokemon in the tier. When given Speed control support, can really devastate opposing teams. Fire Blast hits like a freight train, and Heat Wave is quite spammable. It really hates the Rain matchup, and it can allow Tyranitar to Dragon Dance for essentially free if you aren't running Will-O-Wisp. Zapdos also outspeeds and OHKOes it with Thunderbolt. Water types in general destroy this thing, but that's why you have a teammate, to deal with those worse matchups. It's pretty splashable, but you need to be able to account for Tyranitar and Zapdos.

Salamence
Intimidate support is always good, and Salamence is probably the most well-rounded Intimidator. Special/Mixed Salamence are really dangerous to opposing teams, but I don't think physical is very good. The abundance of Ice moves really hurts its viability, but Intimidate will always be good in Doubles formats. Deoxys-Speed and Gengar are two massive pains for this thing, as well as bulky waters. Of course, these can all be dealt with via your teammates, and supporting Salamence properly can allow it to really dent the opponent's team.
B+ Rank: Pokemon that will consistently contribute, but have competition that may fit better on other teams.

Milotic
The first of many Water types to come. Milotic has great special bulk and passable physical bulk, but what sets it apart is its 100 Sp. Atk stat and access to Hypnosis and Recover. The other main Hypnosis Water type, Politoed, doesn't hit as hard and lacks the speed tier that Milotic has. What Milotic lacks though is an immunity to Explosion, unlike Politoed. The issues with it are fairly plain to see: Bulkier Pokemon are much easier to take down when they can be double targeted in a turn. You want a lot more EVs than you have available to you. You really want Recover, but you also really want all of Ice Beam/Hydro Pump/Protect/Icy Wind/Hypnosis. Despite these flaws, Milotic still performs consistently well. Offensively, it hits like a truck with Hydro Pumps or provides great support with Icy Wind, while being able to pretty easily shut down Rain teams (except for HP Grass Ludicolo).

Suicune
A different kind of bulky water. Instead of a general glue bulky water, Suicune functions as a potent win con. When given the opportunity to Calm Mind, Suicune can very easily tip the game in your favor. The issue therein lies in finding the opportunity to Calm Mind. You are very susceptible to Electrics, Taunt, and Explosions. Once Suicune gets going though, it can be very difficult to stop. Pairing it with support such as Fake Out or Follow Me is not a bad way to make sure Suicune is sustained. It's well worth the support.

Jirachi
Metagross typing with better special bulk, much better speed, and a lot more coverage. It functions more like a wincon than Metagross does, similarly to the comparison of Milotic and Suicune. Jirachi lacks Explosion and a monstrous Attack stat to pound everything like Metagross does, so it is way lower. Instead, it beats everything via Calm Minds and coverage. It is easier to find a time to set up Jirachi than Suicune, but Jirachi doesn't have the Suicune bulk to last as long. It suffers incredibly from 4 moveslot syndrome. You want way too many attacks on this thing.
B Rank: Pokemon that either require support to be wholly effective, or provide effective support.

Celebi
It functions similarly to Jirachi, but it's better/worse against specific Pokemon. It's better against Electrics and Waters, and worse against Steels. Not as splashable as Jirachi, but teams looking for a bit more Zapdos or Water type insurance get a ton of value out of it.

Raikou
Another Calm Mind Pokemon. It's pretty difficult to find a time to get this to set up, as Earthquake is not uncommon, but once you can get that first Calm Mind, you're in business. Thunderbolt is incredibly spammable, and having a resistance to it as well as the ability to spam boosted Thunderbolts is awesome. It also has a great speed tier. It pretty much dies to every Earthquake, though.

Gyarados
The second best Intimidator, with a better support movepool but less offensive presence. Dragon Dance Gyarados is nigh impossible to pull off, but its unboosted physical attacks can do some pretty decent damage. Support wise, it has access to moves like Thunder Wave and Icy Wind, while sporting a pretty beefy Special Defense stat of 100. Gyarados loves Marowak support, and that makes it much more difficult to kill. Gyarados can actually function with a Dragon Dance set if given Marowak support, but outside of it, good luck.

Ludicolo
Outside of being a Rain sweeper that can hit opposing water types, it also functions as a Rain counter with Fake Out, as well as a passable special sponge in general. It can get pretty strapped for move slots, and it should not be your only special check, as it can get worn down pretty easily.

Snorlax
It has the strongest move in the game: STAB Selfdestruct. It won't work as a wincon with curse, but it'll be an effective special sponge for your team. It gets pounded by physical attacks, and can really quickly do middling damage due to Intimidate.
B- Rank: Pokemon that fill specific roles, but they execute those roles very well.

Houndoom
This thing has one job, but it fulfills that one job excellently. Alongside Deoxys-Speed, it smashes the Metagross + Gengar lead combination. While other Fires could beat Metagross just as well (if not better), they don't beat Gengar like Houndoom does. Boomless Gengar loses really hard to Houndoom. It's a pretty specific niche to fill, but it's a pretty great one to fill. Oh yeah, it also beats Superpower-less Deoxys-Speed, and it's immune to Fire.

Dewgong
Never in my life did I think Dewgong would be this high on any Viability Rankings, but here we are. This thing is sick. Fake Out, Encore, Icy Wind, +1. Encoring a slowed Pokemon into Protect for up to 6 turns is awesome, and Fake Out is obviously invaluable. It's just a bit passive, but constant Icy Winds and Encores will absolutely make your opponent angry enough to want to attack this thing. Not a perfect fit on every team by any stretch of the imagination, but the teams on which it fits it does incredibly well.

Exeggutor
It's a Sun sweeper with Explosion and Sleep Powder. It struggles like hell outside of Sun, so be wary.

Kingdra
It's a Rain sweeper. Not much more to add.

Hariyama
The only Pokemon to get both Fake Out and Helping Hand. It has a really beefy Attack stat of 120, and can either absorb status with Guts or sit on Sun teams with Thick Fat. It hates the existence of Salamence, Zapdos, Deoxys-Speed, and the other Psychics. It has a huge problem with moveslots.

Politoed
Politoed is the best answer to the best Pokemon in the tier: Metagross. Damp completely shuts down Explosion from Metagross, as well as the other Exploders in the tier. It also provides support via Hypnosis and Icy Wind, and can fire off a Hydro Pump or two to deal sizable amounts of damage.

Togetic
Togetic gets all of Encore, Helping Hand, and Follow Me. Clefable gets Follow Me and Encore, but I haven't used it enough to gauge its effectiveness. Togetic, on the other hand, is a pretty decent Follow Me user. It has respectable bulk, other good support moves (Thunder Wave, Helping Hand, Encore, Yawn, Dual Screens). Has a real issue with being passive, but Follow Me prevents your opponent from ignoring it.

Marowak
A great team support Pokemon. With the prevalence of Electric types like Zapdos, or Thunderbolts from Gengar and Deoxys-Speed, Pokemon weak to Electric such as Moltres or the abundance of Water types in the tier, being able to redirect Thunderbolts at the cost of no health is fantastic for you. However, the same things that you are redirecting Thunderbolts from destroy Marowak.

Steelix
Steelix does really well vs Metagross and Electrics, and it has Explosion. It REALLY hates Water types, but it can probably eat a Surf and use Explosion to Retaliate. Use Hidden Power Rock over Rock Slide so that you can slam Zapdos.
C Rank: Pokemon that are good on specific teams, but are usually not good fits on most teams.

Dusclops
It has Imprison + Protect to completely stop opposing Protects, allowing free Explosions/attacks. It has a pretty good support move pool as well, in stuff like Icy Wind and Helping Hand. The biggest issue is just how passive it is.

Jolteon
Jolteon provides good weather support to Rain teams with Rain Dance, Helping Hand, and Rain accuracy boosted Thunders. It provides mediocre support on every other team, which other Pokemon can fill.

Lapras
Lapras is a Water immune that also 4x resists Ice Beam. It completely destroys non Ludicolo Rain sweepers, and shuts down a lot of Suicune/Milotic sets. It's a bit weak to Metagross, definitely struggles vs Electrics, and it isn't great vs TTar.

Golduck
Golduck's ability Cloud Nine hard shuts down Rain and Sun teams, and Calm Mind Golduck is a dangerous wincon vs Rain. Cloud Nine also allows you to gain Leftovers in Sand. It has pretty average stats overall, but Cloud Nine gives it a small niche on teams.

Espeon
It's a Psychic type with Helping Hand. Psychic is pretty spammable, except against Tyranitar and Metagross, which is why you have Helping Hand. It's seems pretty mediocre on paper, but it put in a ton of work when I tested it out.

Flygon
I used this thing and I found it incredibly underwhelming. It doesn't hit nearly hard enough for my liking. It does destroy DD Tyranitar, and STAB Earthquake hits with a degree of strength, but it hates all of the ice moves around in the tier.

Vaporeon
It's a Water type with a ton of Sp. Atk and Helping Hand. Usual Water type issues apply.
If I didn't put something on here, I either didn't think it was good enough to be put on here, or I haven't used it enough to form a substantiated opinion on it. This tier is still new, so feel free to try out some stuff I didn't put on here!