Just laddered a bit with psychic for the core challenge, time to put in a few nominations:
Psychic:
B -> S/A
Although this mon is technically not mandatory due to there being other Ghost immunities available (namely, Meloetta), this really deserves at least A and strong consideration for S. This is on 95% of psychic teams for good reason; it's a necessary part of PsyTerrain teams (A major playstyle on this type), and even ignoring those, Psychic terrain's utility in easing revenge killing threats such as Kingambit, Mimikyu, and Azumarill is really nice. In addition, this mon is good standalone, being able to run both Specs and Scarf effectively, and it even has nice utility options such as Trick and Healing Wish. B rank is massively underselling this mon and its importance to the type.
B -> C
How the mighty have fallen. Slowbro lost Teleport and Scald with the transition to gen9. This normally wouldn't be enough to justify a C ranking on this own, but bring a momentum sink for psychic is really dire this gen; losing momentum with this type generally means sacking a mon or worse due to several dangerous threats such as Volcarona, Ceaseless Edge Samurott, Gholdengo, Spectier, and Kingambit, many of which can even setup on defensive mons like Slowbro and sweep the Psychic team or make problems for the Psychic team by setting spikes while getting a kill. In addition, it doesn't scout banded earthquakes (from things like Sand rush Excadrill last gen) effectively becuase psychic no longer has a splashable ground immunity, reducing this mon's value as a regenerator pivot. Overall, defensive pivots without a momentum move are really hard to justify when using a type like this; as psychic currently stands, slowking is more viable than slowbro due to chilly reception alone and that is already in stuck in C. This might deserve a D ranking if we're honest, but possible builds with Slowbro need to be explored more first before coming to that conclusion.
B -> C
Everything written about slowbro above also applies to Glowking; the AV set suffers from the exact same problems. However, it is capable of running HDB with Chilly reception and does absorb Toxic spikes, so there is some hope for this mon to make a comeback.
B -> C
Used this mon during the core challenge and was not impressed. Iron Leaves' main selling points are the speed, water resistance, and grass STAB. That is where the good points end.
Swords dance sets get revenge killed by too many threats (flutter, pult, any fast mon with uturn or first impression, etc) and suffer from four moveslot syndrome (needs Leaf blade + CC; without night slash you lose to Gholdengo, without Psyblade you lose to toxapex/amoonguss/sneasler/Volcarona/Tauros/etc). I would not recommend using a swords dance set.
Scarf sets hit a fantastic speed tier but lack power; outspeeding +1 Volcarona is nice but you don't KO even with terrain support (252 Atk Iron Leaves Psyblade vs. 0 HP / 0 Def Volcarona in Psychic Terrain: 241-285 (77.4 - 91.6%) -- guaranteed 2HKO) and honestly even has problems killing frail targets like Flutter Mane without support (252 Atk Iron Leaves Leaf Blade vs. 0 HP / 0 Def Flutter Mane: 237-280 (94.4 - 111.5%) -- 68.8% chance to OHKO), and also doesn't kill many important targets you'd really like to kill (252 Atk Iron Leaves Night Slash vs. 0 HP / 0 Def Spectrier: 232-274 (68 - 80.3%) -- guaranteed 2HKO after Leftovers recovery, 252 Atk Iron Leaves Close Combat vs. 252 HP / 0 Def Heatran: 248-294 (64.2 - 76.1%) -- guaranteed 2HKO after Leftovers recovery, 252 Atk Iron Leaves Night Slash vs. 0 HP / 0 Def Dragapult: 194-230 (61.1 - 72.5%) -- guaranteed 2HKO). In addition, as good as the speed tier is it still gets outsped by +2 cloyster and Polteageist, so you'll need another answer to these if you don't want to lose to them. Overall while the scarf set is useable it has many flaws and needs a lot of support to shine.
C -> D
If I'm looking for a mono psychic with Calm mind and stored power with Fairy coverage, I'm using a certain oversized turkey before ever touching this thing. Not nominating to UR because Trick room + lunar dance is cool, but that playstyle is really niche and doesn't justify a C ranking.
Also, lastly:
Grass:
C -> UR: Watching Arch play over 180 games on his alt and fail to make bronze has convinced me this is unviable. Does nothing that grass really needs while taking up a valuable teamslot. In architect's words, using this is like playing 5v6. Grassy terrian is nowhere near valuable enough to justify using this; please unrank.