A personal VR doesn't really make a good discussion so lemme double post with reasonings.
Rises
A > S
I used Yveltal 4/5 of my NDWC games. Capable of running both offensive and defensive sets Yveltal is easily one of the most versatile threats there is. So many times the types I was weak to was literally Ground/Dark/Ghost so I found slapping it onto teams really easy. It also checks more Pdon sets than Ho-Oh and is harder to prepare for.
B+ > A
This Pokemon is a THREAT. If you don't have a Blissey it will break everything with ease. A lot of teams counterplay is using Sun and sacrificing Pdon into revenge killing.
B+ > A-
B+ > A-
B > A-
B- > A-
Flutter Mane is a threat and a half and it gets a surprising amount of opportunities to setup. Ferrothorn is probably underranked here. Spikes are quite powerful with all the grounded mons and if you pair it with Rock move SR Pdon you can easily overwhelm Ho Oh with knock off. Webs is underrated and can easy be explored more to see its true power. A lot of powerful pokemon in this tier so Ditto is very reasonable to use and bulkier teams.
(Poison) B- > B+
Underrated form. You lose your ability to check Zygarde and Pdon but you gain the immunity to Toxic and being good Xerneas answer.
B- > B
Best Xerneas and Yveltal answer while providing Volt Switch support.
(Dragon) C+ > B-
I feel like these mons are more than just niche picks. Tera makes Goth quite a potent trapper especially against Scarf Koraidon. Mega Lucario's Bullet Punch sounds super anti meta duel to all of the fast tera ghost users that are usually used for the extreme speeds. Arceus Dragon is Arceus Water that can hit Pdon with it stab so it can use will o wisp better but sadly it loses the ability to check tera fairy Zygarde.
D > C-
I don't know these pokemon being in D feels wrong. They all have interesting qualities that might make them at least somewhat worthwhile.
UR > C-
Got a okay niche with checking Koraidon and some Pdon while being able to preserve your own hazards.
Drops
S > S-
Koraidon is obviously amazing but the ones I have put above are able to be used both offensively and defensively. Koraidon is overkill on the former but the latter is mostly only switching into Yveltal foul play. This is all fine though as this is just my philosophy and most people would disagree with this.
(Ground) S > A+
S- > A+
S- > A+
A lot of this is just cleaning the S rank which I feel is heavily cluttered by Pokemon that are below the big 5™.
Arceus-Ground is great because its a Arceus form that can hit Pdon super effectively but it doesn't offer as many useful resists as lets say Fairy and Dark.
Ho-Oh is great but my god that exploitable weakness really holds it back from S ranks. The most popular SR setter can keep them up vs It very easy. However Ho-Oh is still Ho-Oh and can often times shut down entire threats by its sheer bulk and regen.
Eternatus is great as a offensive threat but its hard to fit sometimes and the Recover nerf hit the defensive sets hard.
S > A+/A
Zygarde is in a weird place due to how good Taunt Arceus forms are and I think it can suffer with being too passive with its low attack. It also doesn't help that in tournaments its getting prepped to shit LOL.
A+ > A
Not gonna lie I didn't see this in the tour at very much. It definitely has a strong niche but I think its low usage comes from inability to tera and lots of the things its supposed to setup on can punish it in some way.
A- > B+
To be honest Marshadow has really suffered with the Speed Creep over the last 2 gens meaning its good trait of Speed is not really something boast about anymore. That and Koraidon making most teams run strong fighting resists makes its 4mms worse.
B+ > B
Its okay. I like its ability its got good bulk and Ghost stab with Moonblast and Focus Blast is obviously broken. I think its problem is that outside of its bulk its a bit worse than the other ghost breakers like Flutter Mane which sports a better speed tier.
C- > D
Whatever weird anti meta thing Kyurem White is trying to is probably done by better by Reshiram whos stronger fire moves can get boosted by Koraidon's sun.