Landorus-Therian: A+ -> S
Landorus-Therian has established itself as one of the most splashable metagame threats so far, and from its incredible ability to pressure every Defogger in the metagame strongly as well as provide a decent defensive role in a high powered package make Landorus-Therian incredibly strong as of now. Teams are almost incapable of utilizing Magic Bounce against Landorus as well, as it is able to bypass every common user apart from Will-o-wisp Moltres, which is a fairly niche pick for balance teams, being difficult to fit on most builds. Explosion Landorus also serves to be the hardest nuke in the entire game, usually claiming a mon with it whilst getting up Stealth Rock at the same time. This makes it an incredibly tough threat to deal with and a primary slot for any sort of offense.
As for what has changed for Landorus-Therian, a more offensive metagame and general liking to a higher speed tier have made it shoot past most of the slower Ubers in the metagame and make it preferable to them as wallbreakers, challenging Primal Groudon considerably. Of course, this does not make Primal Groudon bad by any means and still leaves it to be incredibly splashable due to its differing roles, but Landorus-Therian is the quintessential offensive Ground-type in Mix and Mega for this reason. In terms of synergy, it also enhances Kartana, one of the major threats to teams in the current metagame, where when supported by Landorus-Therian, Kartana can get to grips with taking on usually threatening Ground-types and outright mashing them. Its Flying typing also allows it to avoid Sticky Web, one of the biggest threats in the current metagame as of now, and this gives it a further offensive niche by being able to revenge kill a large majority of rampant Sticky Web threats such as Primal Groudon and Kartana. This also gives Landorus-Therian a defensive role when combined with Explosion as a defogger for bulkier offense teams as previously showcased.
http://replay.pokemonshowdown.com/gen7mixandmega-838936355 This replay showcases the defensive utility of Landorus-Therian and how it can be used in a game against offensive teams to prevent certain team strategies. Both Landorus were capable of stopping a threat from running through the team - in this case, Landorus-Therian stopped Ho-oh from claiming a few on my side and Stresh's Landorus stopped Kartana from getting a kill early game, whilst also setting rocks against my team, chipping down Primal Kyogre during the midgame to help Primal Groudon break (it was revealed to be Rock Polish after the game)
http://replay.pokemonshowdown.com/gen7mixandmega-836875813 Landorus shows a more defensive side that also indicates how splashable it can be on more bulky teams and the sheer power of Aerilate Explosion. It managed to check a threat that was otherwise very difficult to stop, defogged Stealth Rock and then proceeded to trade all in one, doing more than its own slot for that game.
Magearna: A+ -> S (proposed reordering: Latiasite, Pinsirite, Sablenite, Cameruptite, Manectite)
Whereas Landorus-Therian has given offense a boon by being a great role compressor, the already titanically splashable Magearna gives balance a reason to quiver in fear. Its multifaceted applicance to a majority of teams (shown by its incredibly high usage in the Mix and Mega Seasonal as well as >50% usage in WCoP) makes it a terrifying force to predict earlygame and difficult to play around. The sheer preparation this mon causes on builds that aren't straight offense actively disincentivizes defensive counterplay and thus invalidates other previously usable mons in a way that no other Pokemon can really replicate. This isn't just a testament to how much Magearna effects the metagame, but its many roles in a single slot applicable from newer builds such as offensive Latiasite Magearna and Pinsirite Explosion Magearna and general lack of passiveness make it an almost unbeatable slot on current teams. Other Steel-types such as Necrozma-Dusk-Mane and Metagross find themselves in heavy competition for this slot due to the wide range of Pokemon Magearna beats. Whilst offensive pressure from a general team view is the best way to stop Magearna from plowing through teams due to some sturdy checks such as Primal Groudon, for bulkier teams it is quite difficult to execute this and often an entire team becomes weak to another threat simply because they let a variant of Magearna in too often, such as Latiasite and Pinsirite versus Arceus-Ground and Buzzwole teams, Flash Cannon variants of Cameruptite from Tapu Lele teams and Sablenite against Darkrai teams. There have been adaptations to counter this, such as Ampharosite Milotic on balance, but in general it does not stop the many roles of Magearna from being passed up, and teammates can be engineered to pressure this, such as SubToxic Primal Kyogre. It is also one of the few Pokemon capable of stopping Pinsirite lures such as Terrakion and Kartana from sweeping without passivity, making it an excellent fit as a pivot on -atespeed bulky offense.
However, this is in a vacuum, and most games are through offense. However, this does not mean that Magearna is deadweight:
http://replay.pokemonshowdown.com/gen7mixandmega-836886959 This replay shows how Magearna can use its many sets to its advantage and bluff a set, making it particularly brutal for Ground-types other than Primal Groudon to deal with it properly. Not only this, but it very significantly damaged another Manectite Magearna, giving Victini very good initiative, and had Magearna not been chipped, it could threaten it out with a Volt Switch or damage similarly with Thunderbolt, which would cause issues for the team. Cameruptite Magearna also managed to 1v1 a Lunala in this game despite lacking general switches. Manectite Magearna then continued to stay significantly annoying for the remaining Zeraora, requiring it to predict its switchin and chipped Victini into Kartana's range.
http://replay.pokemonshowdown.com/gen7mixandmega-842619600 Thanks to the previous Landorus switchin, Magearna is capable of taking an Explosion to force a switch itself, and then stops a Pinsirite Magearna from winning from then. The opposing Pinsirite Magearna then takes on from the defog Landorus, otherwise a big threat to the webs, manages to tank a Z move from Lunala, and then trades with it, opening up a potential breaking spot for the remaining members. However, Entei managed to stomach the opposing Z move and win. This showcases how easy it is for Magearna to find setup opportunities and change the game, despite being on offensive teams, and shows how difficult it can be to stop a Magearna from snowballing, in particular Pinsirite, due to how it exploits common Ground-type checks.
http://replay.pokemonshowdown.com/gen7mixandmega-829206426 Magearna simultaneously shows off its power and utility at once by dealing 53% to a Primal Groudon after forcing out a Tapu Lele through a bluff, allowing Mew to check Primal Groudon after a Swords Dance without taking heavy chip. The battle then continues to go on until it ends with Magearna checking Tapu Lele. Despite the general age of this replay, it still shows how the multifaceted and versatile Magearna can make bluffs with a lot of Pokemon, often times itself to clear out the way for another Pokemon to clear up.
Lunala: A -> A+
I said no to this last update, however, as the metagame has been going onward, Lunala prep has become scarcer and harder to fit on a team. Darkrai, previously the best revenge killer of Lunala, is heavily plagued by Magearna and Altarianite Zygarde's dominance, making it a significantly worse breaker than other fast wallbreakers such as Kartana and Terrakion. As well as this, offense generally lacks the capability to stomach the Z move very well, as well as consistently take Moongeist Beams after the Z has been burned, especially from Calm Mind + Roost sets that have arisen due to the lack of sturdy resistances on teams. However, this is not the main appearance of Lunala. Its main role in MnM is as a particularly brutal wallbreaker on Sticky Webs teams that is capable of providing a general blanket check to a majority of Sticky Webs threats such as Altarianite Zygarde and Entei. It also patches up Darkrai as a major weakness of Lunala, leaving only Mandibuzz / sablenite Magearna to handle it.
However, if you wish not to take that path, Lunala is also capable of running a very powerful Defogging set with its Z move, being particularly nice for bulky offense as it allows Lunala to take on and properly punish both Landorus-Therian and Primal Groudon, something no other Defogger is capable of doing reliably.
https://replay.pokemonshowdown.com/gen7mixandmega-833355974 Lunala sweeps after two Magearnas collide and annihilate. In this particular case, the opponent was fairly reckless with their Flying resist, and let their secondary resistance (and thus resistance to Lunala) get taken out before they were able to do anything to Lunala, leaving it an easy job to clean up.
http://replay.pokemonshowdown.com/gen7mixandmega-827078050 Lunala claims two Pokemon from lead, letting the opposing team get a significant advantage due to the matchups these Pokemon had. In particular this shows how Lunala's Shadow Shield allows it to act as a secondary lead on Hyper Offense teams, to such a degree that it is almost ubiquitous on these sorts of builds.
Zeraora: B+ -> A-
Zeraora has recently become one of the rising stars of the current metagame, and this is mainly due to all of its niches contributing to a good mon against the offensive metagame. Zeraora is actually bulky enough to take on Flying types, making it a prime offensive pick against Sticky Web teams as well as a compression of speed control whilst still being relatively powerful. In some cases it's what Weavile wanted to be as a mon - having some utility really helps it get in switches and take down its low maintenance checks such as Primal Groudon and Magearna, and its through this pressure that grants Zeraora a better ranking than it should have right now. Not really much else to explain about Zeraora, rather than its underrated and adapts to the meta better than what has been shown.
http://replay.pokemonshowdown.com/gen7mixandmega-827634518 Zeraora's Electric typing allows it to take advantage of a rising anti Webs tactic in Noivern and, due to the opponent's removal of Magearna as its answer, Zeraora cleans up the entire team.
Toxapex: A- -> B+
Toxapex has been relatively unseen in tournaments for the past few weeks, and for good reason. The more offensive metagame and adaptation of Toxapex's checks to its Latiasite set have been quite harsh, such as Landorus-Therian going back to running Salamencite, decreased usage of Arceus-Ground, beampower Mew and Buzzwole, and a greater usage of Smack Down Primal Groudon, preventing it from checking a single threat on Sticky Webs teams at all. Not only this, but it is fairly vulnerable to Kartana despite Scald's burn chance, and this can make it fairly difficult to fit on a team due to overlapping roles with other checks and the general issue of letting up Stealth Rock from the best setters in the metagame. It absolutely despises higher Tapu Lele usage and viability as well, and overall, the Latiasite set does not work as well as it used to defensively.
However, even the Gyaradosite set has been under the weather, with decreased time to set up hazards due to increased viability of Landorus-Therian and Kartana, and, yet again, the move towards a more offensive metagame. It is unlikely that Toxapex is able to set hazards in a single game due to this issue. The usage of Lucarionite Tapu Lele also means that Toxapex is less effective as a Psychic resist on most teams, and thus is fairly difficult to fit on a team in the current metagame. The combination of these glaring flaws is a sign that Toxapex should drop.
Some lower tier nominations:
Skarmory: B+ -> B-
Skarmory kinda sucks now thanks to high prominence of Magearna usage as well as a generally more offensive metagame making it far harder to operate properly as a Defogger. The Pokemon it was capable of beating previously generally have synergy that allows the metal bird to be easily abused by teammates, such as Double Fire and Double Ground cores, and the usage of Pokemon that generally discourage its usage such as Primal Kyogre and Zeraora. It genuinely fails as a Flying resist in the current metagame and has little place on bulky offense over Noivern as a Fire resistance.
Zapdos: B+ -> B
Zapdos finds itself in a very awkward position in the current metagame where it actually does nothing to some things it's expected to check, such as Landorus-Therian and Zeraora. Whilst it is also arguably one of the best Kartana checks in the metagame, it is also one of the worst momentum drains as a Defogger this way due to it being inable to properly take down Ground-types, especially Zygarde. It also does not block Stealth Rock from Swords Dance + Stealth Rock Primal Groudon, requiring support from the likes of Landorus-Therian to cover for the loss of momentum offensively, meaning that as a Flying-type its role is hindered. Whilst it does not suffer as badly as Skarmory, it certainly hates the Landorus centred metagame.
Keldeo: B -> B-
No real reason to use this mon over Terrakion. Hasn't been used in a long while and generally suffers from higher Altarianite Zygarde usage as well as a more offensive metagame with a greater usage of Webs there to kill its only niche over other water breakers such as Primal Kyogre.