I have been very inactive due to school stuff and now I'll be inactive again, sorry
Well, before I go on vacation I'll make some noms:
Rayquaza-Mega from A+ to S
Rayquaza has proven to be one of the most dangerous and versatile Pokémon in the Ultra series metagame. Its wide movepool, in combination with incredible stats and a helpful ability makes Mega Rayquaza a Pokémon to keep in mind when teambuilding.
Mega Rayquaza is broken because there isn't a single Pokémon that can counter every single one of its sets, and you will almost lose a Pokémon in the process of scouting. Incineroar is not a switch-in, as a -1 Choice Band-boosted Dragon Ascent 2HKOes it, and if Rayquaza is AV, Incineroar will lose a lot of health with Earth Power. Groudon-Primal also suffers from Earth Power, and has to be careful for a Water-type move like Waterfall. Lunala can't OHKO Rayquaza with Menacing Moonraze Maelstorm if Rayquaza is AV, and Crunch can 2HKO it (Adamant Crunch OHKOes it).
We have seen new Rayquaza sets appear, like Icy Wind AV (from Wolfey's NAIC team), or Pinch Berry SD, that takes advantage from the opponent scouting with a free SD. There are even more possibilities, like Draco Meteor, Earthquake, Overheat, Tailwind... And I'm not even mentioning its ability, that can take surprise KOes with Kyogre/Fini or reduce damage from Water Spout/Eruption/etc.
However, Mega Rayquaza suffers from the 4MSS and is not such a threat when its set is revealed, as the opponent will know how to deal with that specific Rayquaza. Xerneas is also a big threat for Rayquaza, but as long as you have a good Xerneas counter as its teammate, you're OK with it.
Ferrothorn from C to B?
Don't get me wrong, Ferrothorn sucks and is the worst Pokémon of all time. But I think that C is too low for Ferrothorn. For example, C-rank Pokémon are Zygarde (lul), Alolan Persian or Suicune. Even B- rank has HItmontop or Gothitelle, which don't see much usage nowadays.
Yes, Ferrothorn loses all of your momentum, but it's a great tanky mon that has some type of offensive pressure with Gyro Ball or Power Whip. Its Incineroar weakness can be covered with Primal Kyogre, and Ferrothorn is a great switchin for Tapu Koko or Xerneas which otherwise would be difficult for Kyogre to deal with.
Other minor noms:
Tapu Koko from A- to A (has proved to be really good)
Cresselia from B to C/C2 (unmon)
Manectric-Mega from B+ to B/B-
Crobat from B to B+
Hitmontop from B- to C2
Nihilego from UR to B/B+
Kommo-o from UR to C/B-
Solgaleo from C to B- (won Japan Nats but I think that's all)
Lopunny-Mega from C2 to B- (really cool offensive mon)
Tornadus from C2 to B (is a metagame staple)
Shedinja from UR to C/B- (why is this mon UR?)