Alternate forms: Zygarde-C in banned, Tornadus-T in banned, Landorus-T in great, Zygarde-10% in decent, Thundurus-T in niche
Here are my speculative thoughts on where all the returning Pokemon will stand in relation to the OU metagame.
Disclaimer: all my thoughts listed here are purely speculative.
Banned:
Genesect is still ridiculously good with the extremely wide variety of sets it can viably run, quickban.
Expanding Force in Psychic Terrain is just way too overwhelming, quickban.
Pheromosa now has access to Heavy-Duty Boots to be an even better pivot, quickban.
The lack of Z-moves hurts it, but it still has insane coverage and the speed boost from Beast Boost in tandem with its 4x resistance to Grassy Glide probably makes it way too difficult for offensive teams to handle, quickban.
I think Blaziken should definitely get its fair chance in OU now that it lacks access to Z-moves. However, similarly to Nagandel, Speed Boost in tandem with Grassy Glide resistance spells doom for offensive teams, quickban.
Similarly to Tapu Lele but not quite as broken, Rising Voltage in Electric Terrain just seems too overwhelming, quickban.
Tornadus-T now gets access to Heavy-Duty Boots and Nasty Plot, which makes it both a ridiculously good Regenerator pivot and a major offensive threat. I think this Pokemon will be quickban-worthy too.
Landorus-I is still a really good wallbreaker, but maybe with Corviknight around it might stay around for a while. Still think it's probably going to be quickban-worthy though.
Similarly to last generation, I believe Zygarde will be able to run a variety of different viable sets, which makes it too hard to prepare for and pushes it over the edge, and now the lack of HP Ice benefits it too. If you're running Corviknight, Zapdos, or Moltres as your Ground immunity, good luck with Zygarde. However, as TPP has brought up, Zarude might actually make for a solid check to it since it can Bulk Up, use Jungle Healing to shake off Glare/Toxic, and hit through Zygarde's Coil boosts with Darkest Lariat. I think Zygarde is probably going to be banworthy, but might have to go through a suspect test first.
Great:
I bet Landorus-T is once again going to be the most ubiquitous Pokemon in the metagame. Electric immunity, ground immunity, Intimidate, great Attack stat, good bulk, decent Speed, wide movepool including Stealth Rock and Defog, and so on make it a great fit for a wide variety of teams. The lack of HP Ice makes it even better, as it can now safely wall the vast majority of Electric-types (as well as opposing Landorus-T funnily enough). Who knows, perhaps this Pokemon could finally be on the chopping block?
I believe Heatran will be at the top of the metagame. It matches up very well against the currently ubiquitous Clefable, Corviknight, and Toxapex. Additionally, with Gliscor not returning, there are barely any switch-ins it cannot punish with a set of Magma Storm/Earth Power/Toxic/Taunt, provided it doesn't miss of course. The lack of HP Ground also means it stonewalls Volcarona.
Even though it doesn't have a Z-move to use, I believe Garchomp will be fantastic. It's a Stealth Rock setter that matches up well against some very important Pokemon, namely Heatran and, thanks to the lack of HP Ice, Zapdos and Moltres, meaning it can keep up Stealth Rock against these birds.
Zapdos benefits a lot from the addition of Heavy-Duty Boots, which I believe will make it one of the primary Defoggers (if Tornadus-T gets the boot at least). The loss of HP Ice does mean it will struggle against Ground-types like Garchomp though.
Moltres benefits even more from Heavy-Duty Boots and now gets Scorching Sands, which will allow it to check Heatran and keep its Stealth Rock off the field. Of course, its glaring weakness to Knock Off and Trick is a fairly big downside.
No Pursuit? No Greninja? Heavy-Duty Boots? Access to Scorching Sands to blow back Heatran and burn Tyranitar? Blacephalon absolutely loves the new generation. I'm excited to see how it will turn out.
Kartana is amazing in SM and I think it will be a great Pokemon in SS as well. The lack of Z-moves keeps it in check, but the fact that it can fire off stuff like Choice Band-boosted Knock Offs into would-be checks like Corviknight, Zapdos, and Moltres is scary.
I think Victini will be better than ever. Heavy-Duty Boots should make it a really nice pivot and Choice Band or Choice Scarf with Trick promises to be great as well.
The fact that it's one of the best checks to Heatran and Hydreigon is really big. However, the lack of reliable recovery and Defog removing its terrain is not ideal.
VinCune is the result of Primarina and Kyurem doing the fusion dance, while CroCune is probably one of the most reliable Heatran checks out there, so I think Suicune's going to be really good.
Heavy-Duty Boots make it guaranteed that the impact of the first hit it takes is going to be halved by Multiscale, which should allow for some fairly safe Dragon Dance setup. Combined with a new reliable STAB in Dual Wingbeat, I believe Dragonite will be a scary sight, especially to offensive teams.
The lack of Pursuit and the addition of Aura Sphere and Mystical Fire to its arsenal should make Latios quite good. It makes for a fairly solid Heatran check if anything.
Grassy Glide Tapu Bulu is obviously great, but I'm not sure how well it can compete with Rillaboom, which has access to U-turn and Knock Off and is faster. Tapu Bulu does have that Fairy typing as well as Horn Leech and a newly acquired Play Rough though, which could potentially set it apart I suppose.
Decent:
Thanks to its resistances to Fighting and Dark and fantastic physical bulk, Buzzwole makes for quite probably the most reliable Urshifu-S check out there.
With Sheer Force-boosted Sludge Wave, Earth Power, Ice Beam, and Flamethrower, Nidoking will be able to tear apart currently ubiquitous Clefable/Toxapex/Corviknight cores.
I actually think Raikou is quite slept on. It's an Electric-type that doesn't suffer from the loss of HP Ice very much, as it has acquired Scald as a new move, which is a great weapon against Ground-types like Landorus-T. Pressure is also useful. To me it seems like a better version of Calm Mind Zeraora.
Be it as a Choice Band wallbreaker, Trick Room cleaner, or Stealth Rock setter, I'm certain Stakataka can find a decent spot in the metagame. The biggest things that hold it back are its 4x weaknesses to both Fighting and Ground.
While somewhat outclassed by Corviknight as a defensive Steel bird, I believe its superior offenses can help it find a decent niche.
Healing Wish and perhaps even Wish should give this a spot over Latios on certain teams.
Heavy-Duty Boots make Volcanion a lot better. I think it could be quite a solid Defog user, as Steam Eruption and Flamethrower cover quite a lot of hazard setters.
Azelf can function as a more offensive version of Nasty Plot Mew, which I believe to be fairly promising. Mew definitely does the hazard setter job better though.
Teams that rely on Corviknight, Skarmory, Zapdos, or Moltres as their Ground immunity will have a lot of trouble with Zydog.
Niche:
With moves like Heat Wave, Superpower, and Grass Knot, Tornadus actually has solid coverage without the need for Hidden Power. Prankster Defog could also be nice with Heavy-Duty Boots.
With Heavy-Duty Boots to use, I think you can get fairly creative with how to use Salamence. The combination of Dragon Dance and Moxie is of course terrifying, but the lack of Z-moves might make it hard to get the ball rolling.
Heavy-Duty Boots boosts Articuno from garbage to usable, but I think its typing still holds it back. Ice doesn't give it any additional resistances over Zapdos or Moltres, since the only thing Ice resists is Ice itself and it's part Flying. Great special bulk, Heal Bell, and Pressure is a very nice combination though, so I could see it having a niche on stall teams.
Thundurus-T is tremendously strong, but the loss of HP Ice (and Flying) hurts it a lot, as it now struggles to get past Landorus-T. Heavy-Duty Boots are nice for it though.
Same story as above but it can pull off some annoying Prankster Thunder Wave stuff too. A Defiant user that doesn't lose to Corviknight can potentially be nice as well.
Hits even harder and has a better boosting move than Thundurus, but it has less speed and defensive utility. The loss of HP Ice hurts a lot, but Energy Ball is neat.
With Heavy-Duty Boots and Sacred Fire I could see Entei having some niche defensive utility, akin to Arcanine and Flareon earlier this generation.
Nihilego can set Stealth Rock and even Toxic Spikes, while threatening Clefable and certain Defog users like Zapdos. The fact that a bunch of Fire-types like Moltres and Blacephalon get Scorching Sands now kinda hurts it though.
As we've already learned from Seismitoad and Gastrodon, Water/Ground has a lot of defensive utility as a typing. Stealth Rock and having a very favorable matchup against the likes of Heatran and Zapdos can give it a nice niche.
Cresselia is most likely going to be a staple on Trick Room teams again.
Uxie is a staple lead on such teams thanks to Stealth Rock + Trick Room + Memento.
Metagross could be an okay Choice Band user with access to Trick, Explosion, and priority in Bullet Punch.
Registeel might have enough defensive utility to be a niche Stealth Rock setter.
Being one of the fastest Taunt users out there could give it a niche over other hazard leads. It also gets Dragon Dance now, which is neat.
Unviable:
I for one welcome our Heavy-Duty Boots and Pressure overlords.