Mega Absol B+ -> B
Now that Weavile is in the tier, Mega Absol has a lot more competition for an offensive Dark-type. With Buzzwole in the tier, Mega Absol was actually a decent Dark-type due to its ability to lure in Buzzwole and 2HKO it with Fire Blast, while Weavile/Krookodile struggled to get past it. Mega Absol's 4MSS is probably a lot more prominent now that Buzzwole is gone, since many Fighting-types (Bewear, Cobalion, Terrakion mainly) will see the limelight again, forcing Mega Absol to have to fit in moves like Play Rough / Fire Blast to deal with them as well as potentially Iron Tail for Fairy-types, that are creeping in popularity with Clefable's departure (Sylveon and Florges specifically) while also wanting both Knock Off and Sucker Punch the majority of the time. The competition for the mega slot is also a lot more prominent as the top tier mons in UU are primarily mega evolutions so it is a lot harder to fit Mega Absol onto teams, especially since there are similar-ish options with Weavile, Krookodile, and even A-Muk. Also, I don't see Mega Absol on the same level as I do Nidoking and Suicune sooo yeah... However, I do not believe Mega Absol should drop any further since it has ways to lure in or get around its counters which sets it apart from its competition, ultimately forcing the opponent to play around and scout its set and Magic Bounce is still pretty potent as well.
Arcanine C- -> D
Simply put, Arcanine is probably the new meme of UU (and it will probably stay in UU since it is the patron/god of newer players) since Mega Audino and Pelipper or going to drop or rise (in Pelipper's case). While Arcanine has solid stats, its movepool and Fire-typing are really the hindrances of this mon. Offensively, Arcanine is outclassed by both Infernape and Entei (and Darmanitan in some respect) so there is really no reason to run offensive Arcanine. Now with that aside, being a wall that have moves that inflict recoil (Flare Blitz and Wild Charge) or lower its defenses (Close Combat) is counter-intuitive for a Pokemon that is supposed to be a wall. While it can utilize Flamethrower/Fire Blast instead, Arcanine ends up being more vulnerable to the tiers multitude of viable bulky Waters (Suicune, Alomomola, and Jellicent are decently popular-ish nowadays), which can typically force Arcanine out or just beat it 1v1. Anyways, there really isn't a reason to use Arcanine anymore honestly. It loses to basically every relevant wall in UU (bar Amoonguss and Blissey in some cases), while providing very little utility besides walling a very small handful of mons (namely Beedrill and Cobalion), which is prominent now that Buzzwole is gone.
Mamoswine A- -> A
Mamoswine is pretty terrifying atm with Buzzwole's departure. Prior to the council vote, many people used Buzzwole as one of the very few, viable Mamoswine switch-ins, which inadvertently caused a very small influx in its HP Flying set as a lure to it. With Buzzwole gone, Mamoswine has a very limited amount of Pokemon that can reliably switch in to it, ultimately causing it to be a pretty scary wallbreaker (I can see Mamoswine measuring up to Primarina as of now). The Pokemon that can sorta switch in to it (Slowbro and Alomomola) also tend to not appreciate Knock Off and in rarer cases, Freeze Dry. Mamoswine's Stealth Rock set is also pretty decent as well since it can take advantage of its ability to force a good amount of the tier out in order to set them up.
Hawlucha B- -> B/B+ Agree
A lot of people summed up my thoughts so yeah... I'll leave it at that
edit- I feel like we/you should consider ranking the mons from A- and up based on viability (rather than alphabetical order) so we could drop a mon within their subrank rather than an entire subrank, if that makes any sense. There are also clear winners/underdogs in each subrank (for example, Terrakion and Mega Sharpedo are the underdogs of the A rank but, I don't believe they should drop an entire subrank while Mega Pidgeot and Conkeldurr are probably the standouts of the A rank).