Mid-OMPL noms. Also going to be in some detail covering my thoughts in these noms. You can find my general meta observations in this post on the main thread.
Rises:
(Imposter) S -> S+
Imposter has shown its strength and it is quite a bit above any other mon in terms of viability. This can be an unnecessary mon considering nothing else is S- but this is a S+ imo.
A- -> A
Flutter Mane has two primary sets being Flare Boost and Pixilate, and these have very different checks (good thing Flare Boost reveals itself so you don't need to predict which set it is). These sets are also very very good, with Flare Boost counterplay generally being either Scales Zacian or SpD pivot into revenge-killer, which is unreliable considering Tera, and Pixilate just boasting absurd damage output. Flutter Mane also has seen good popularity in OMPL in 3rd usage and a >50% winrate.
(Imposter Cloak) A- -> S-, A+ at worse
I think that Cloak Imposter is like A+ min by virtue of being an Imposter that has benefits over Eviolite. Cloak blocks only 4 main moves - Mortal Spin, Salt Cure, Nuzzle, Spirit Shackle, but these moves are so common on defensive mons as ways to punish Imposter (especially Mortal Spin for bulkier teams) that Cloak Imposter has a notably better matchup into fatter teams than Eviolite Imposter. This makes Cloak Imposter a strong consideration for teams that want a better/shore up a weaker matchup against bulkier teams.
B -> B+
Iron Bundle has two main niches: outspeeding and revenge-killing Flutter Mane and Koraidon, and having a STAB that is rarely resisted on common Ice Scales mons. This makes Iron Bundle a very potent wallbreaker due to Boomburst outpping Recovery and the amount of opportunities Iron Bundle can get to click Boomburst. It still suffers from middling damage output without Tera and vulnerability to hazards.
B -> A-
Dondozo is the best check to Koraidon due to massive physical bulk and a resistance to Flip Turn. Its passivity is also less of an issue when it has one of the slowest pivots, notably underspeeding Ting-Lu. Due to not requiring a lot of moves other than Flip Turn and recovery, it can also fit techs to check other threats like Groudon.
B- -> B
Fireceus has seen some good uses in OMPL with DesoLand having some neat improofing capabilities while the strong Fire STAB in general making it a great Mortal Spinner. Has some serious issues against some walls like Imposter, Dondozo, and Eternatus though.
B- -> B
Arceus-Steel has seen some use as a SpD with good speed tier and typing against the -ate mons. It notably has SE STAB against Flutter Mane while having innate immunity to Mortal Spin and Toxic.
B- -> B+/A-/A
Rayquaza has shown its absurd potential with next to no good answers, defensive or offensive. It also provides a lot of utility with its extremely powerful Espeed and good amount of opportunities to switch in with the solid Flying-type.
C -> B-
I think unlike the stuff in C Gholdengo is a lot more consistent. Provides a lot of team utility through improofing and potentially soft checking some stuff while having great offensive potential.
D -> C
Call me biased but Kyogre is not on the same tier as the stuff in D and is not all theorymon either.
(Imposter - Plate) D -> C
Imposter that has the slightest niche is better than the stuff in D.
D -> C
Has seen some use and success due to its amazing typing, decent bulk, and offensive potential and speed tier.
D -> C
Meloetta has seen use as a SpD sponge that has a decent matchup into Flare Boost Flutter. Its Psychic STAB is also surprisingly useful against threats like Poison-types. Future Sight is always a possibility too.
D -> C
Toxapex is very slow and has a decent typing, and with usage should probably be lifted out of the D.
Drops:
A+ -> A
There is less reason to use Ghostceus since the Slaking ban as Koraidon is spamming Dragon STAB. It also has some very iffy matchups such as into base Chansey, Scales Zacian, and Meloetta. It is still very good but I don't think its on the same tier as the (only) best physical wallbreaker in the tier that has like no switch-ins outside of getting predicts right.
A -> A-
Groudon kind of just does nothing, which can to a degree be reflected through its winrate. As a FC mon it fails to check Koraidon due to Flip Turn weakness. Offensively people are always preparing for it and there are matchups where it just does nothing. Meanwhile it lets the biggest threats in: Koraidon Flips, every other special attacker threatens OHKO, Ray is immune to Ground.
B+ -> B
Chansey is generally prepped for by any special attacker. It is also immensely passive which can easily be taken advantage of by utility mons or physical/mixed breakers. Also comes with the opportunity cost of not being able to use the best mon in the tier, Eviolite Imposter.
B+ -> ? (B or lower)
Wildcard mon, has seen 1 total use in OMPL. Very fishy mon because you can still get the right matchup and just win but otherwise the mon straight up does nothing. Generally just worth using something else.
B -> B-
Generally not as useful anymore because Water isn't that threatening offensively. Dondozo also does a lot of the roles while not consuming the Arc slot. Sure you hit Groudon very hard but Dozo just flip turns anyways.
B -> B-
Enamorus has seen very little use, likely due to its weakness to Stone Axe and Stealth Rock, combined with its general neutral typing and lowish bulk makes it hard to actually bring it in, especially with its tendency to not straight up OHKO a lot of threats.
C -> D
Slaking is gone, there is still probably niche in using Origin over say Etern or Altered.
C -> UR
Its very hard to utilize Pikachu because you need to fish for the right tera type on Pikachu. In case you don't know, Light Ball is the same boost as Choice item/Flame Orb ability + Tera. This means teams are already prepping for that damage level with their defensive walls, making Pikachu far less threatening. Also if you paraed the opposing threat you are already in great shape.
C -> D/UR
These mons are basically unviable offensively and outclassed.
D -> UR
The issue here is that its weak to Rocks, but you also want a way to deal with poison from Groudon. Flying STAB also doesn't hit a whole lot from 120 offenses.
D -> UR
Slaking is banned.
D -> UR
See: Palafin, Ursaluna
D -> UR
Just use another type, Grass is horrible.
Rises:
(Imposter) S -> S+
Imposter has shown its strength and it is quite a bit above any other mon in terms of viability. This can be an unnecessary mon considering nothing else is S- but this is a S+ imo.
A- -> A
Flutter Mane has two primary sets being Flare Boost and Pixilate, and these have very different checks (good thing Flare Boost reveals itself so you don't need to predict which set it is). These sets are also very very good, with Flare Boost counterplay generally being either Scales Zacian or SpD pivot into revenge-killer, which is unreliable considering Tera, and Pixilate just boasting absurd damage output. Flutter Mane also has seen good popularity in OMPL in 3rd usage and a >50% winrate.
(Imposter Cloak) A- -> S-, A+ at worse
I think that Cloak Imposter is like A+ min by virtue of being an Imposter that has benefits over Eviolite. Cloak blocks only 4 main moves - Mortal Spin, Salt Cure, Nuzzle, Spirit Shackle, but these moves are so common on defensive mons as ways to punish Imposter (especially Mortal Spin for bulkier teams) that Cloak Imposter has a notably better matchup into fatter teams than Eviolite Imposter. This makes Cloak Imposter a strong consideration for teams that want a better/shore up a weaker matchup against bulkier teams.
B -> B+
Iron Bundle has two main niches: outspeeding and revenge-killing Flutter Mane and Koraidon, and having a STAB that is rarely resisted on common Ice Scales mons. This makes Iron Bundle a very potent wallbreaker due to Boomburst outpping Recovery and the amount of opportunities Iron Bundle can get to click Boomburst. It still suffers from middling damage output without Tera and vulnerability to hazards.
B -> A-
Dondozo is the best check to Koraidon due to massive physical bulk and a resistance to Flip Turn. Its passivity is also less of an issue when it has one of the slowest pivots, notably underspeeding Ting-Lu. Due to not requiring a lot of moves other than Flip Turn and recovery, it can also fit techs to check other threats like Groudon.
Fireceus has seen some good uses in OMPL with DesoLand having some neat improofing capabilities while the strong Fire STAB in general making it a great Mortal Spinner. Has some serious issues against some walls like Imposter, Dondozo, and Eternatus though.
Arceus-Steel has seen some use as a SpD with good speed tier and typing against the -ate mons. It notably has SE STAB against Flutter Mane while having innate immunity to Mortal Spin and Toxic.
B- -> B+/A-/A
Rayquaza has shown its absurd potential with next to no good answers, defensive or offensive. It also provides a lot of utility with its extremely powerful Espeed and good amount of opportunities to switch in with the solid Flying-type.
C -> B-
I think unlike the stuff in C Gholdengo is a lot more consistent. Provides a lot of team utility through improofing and potentially soft checking some stuff while having great offensive potential.
D -> C
Call me biased but Kyogre is not on the same tier as the stuff in D and is not all theorymon either.
(Imposter - Plate) D -> C
Imposter that has the slightest niche is better than the stuff in D.
D -> C
Has seen some use and success due to its amazing typing, decent bulk, and offensive potential and speed tier.
D -> C
Meloetta has seen use as a SpD sponge that has a decent matchup into Flare Boost Flutter. Its Psychic STAB is also surprisingly useful against threats like Poison-types. Future Sight is always a possibility too.
D -> C
Toxapex is very slow and has a decent typing, and with usage should probably be lifted out of the D.
Drops:
There is less reason to use Ghostceus since the Slaking ban as Koraidon is spamming Dragon STAB. It also has some very iffy matchups such as into base Chansey, Scales Zacian, and Meloetta. It is still very good but I don't think its on the same tier as the (only) best physical wallbreaker in the tier that has like no switch-ins outside of getting predicts right.
A -> A-
Groudon kind of just does nothing, which can to a degree be reflected through its winrate. As a FC mon it fails to check Koraidon due to Flip Turn weakness. Offensively people are always preparing for it and there are matchups where it just does nothing. Meanwhile it lets the biggest threats in: Koraidon Flips, every other special attacker threatens OHKO, Ray is immune to Ground.
B+ -> B
Chansey is generally prepped for by any special attacker. It is also immensely passive which can easily be taken advantage of by utility mons or physical/mixed breakers. Also comes with the opportunity cost of not being able to use the best mon in the tier, Eviolite Imposter.
B+ -> ? (B or lower)
Wildcard mon, has seen 1 total use in OMPL. Very fishy mon because you can still get the right matchup and just win but otherwise the mon straight up does nothing. Generally just worth using something else.
Generally not as useful anymore because Water isn't that threatening offensively. Dondozo also does a lot of the roles while not consuming the Arc slot. Sure you hit Groudon very hard but Dozo just flip turns anyways.
B -> B-
Enamorus has seen very little use, likely due to its weakness to Stone Axe and Stealth Rock, combined with its general neutral typing and lowish bulk makes it hard to actually bring it in, especially with its tendency to not straight up OHKO a lot of threats.
C -> D
Slaking is gone, there is still probably niche in using Origin over say Etern or Altered.
C -> UR
Its very hard to utilize Pikachu because you need to fish for the right tera type on Pikachu. In case you don't know, Light Ball is the same boost as Choice item/Flame Orb ability + Tera. This means teams are already prepping for that damage level with their defensive walls, making Pikachu far less threatening. Also if you paraed the opposing threat you are already in great shape.
C -> D/UR
These mons are basically unviable offensively and outclassed.
The issue here is that its weak to Rocks, but you also want a way to deal with poison from Groudon. Flying STAB also doesn't hit a whole lot from 120 offenses.
Slaking is banned.
D -> UR
See: Palafin, Ursaluna
Just use another type, Grass is horrible.