I hit top 500 on the ladder last night, so I figured I'd share my thoughts.
Here are the teams I used for context:
team 1 (pokepast.es)
team 2 (pokepast.es)
Feel free to use them, but I'm not a great teambuilder so your mileage may vary.
Palafin and Flutter Mane are the clear top threats in my opinion, but otherwise nothing has seemed too unmanageable.
Flutter Mane has been surprisingly easy to handle, but that's with the AV Iron Tusks set on both teams which can switch into Flutter Mane and KO it before it can KO Iron Tusks, as well as multiple forms of priority as a backup option. I don't think needing to run that many things for one pokemon is healthy for teambuilding, but they are all useful for other pokemon as well. Most special attackers are stopped by AV Iron Tusks (which also hits hard, has knock off, can sweep teams with rapid spin speed boosts, and can maintain momentum with volt switch), and priority is incredibly useful for stopping all the frail fast paradox forms, Chi Yu, and Chien Pao. Booster Energy (in general and on Flutter Mane) has seemed underwhelming so far, but maybe I'm not running into people using it well. If you can force a switch it's a wasted item slot, and I've found being able to switch in and out with and still have the speed with Scarf really nice.
For Palafin, I didn't think the choiced set was particularly hard to deal with, but the bulk up set is absurd. It felt like the only times it didn't win gamse for me were ones where opposing bulk up Palafin got set up first. It's pretty easy to activate hero mode, and extremely easy to Bulk Up. Shed tail isn't needed at all to get the Bulk Up, but it's nice when it happens.
Speaking of Shed Tail, I don't understand the talk of banning it at all. Cyclizar is just not good, it's not really that fast as far as new pokemon go and can't handle priority at all. If it does outspeed, just attacking tends to break the sub right away, at which point it's a just another pivot move but with a huge cost. Dragapult isn't being used that much at the moment with all the new toy syndrome, but infiltrator ignores the sub completely as well, and being the fastest unboosted thing around is great too. Orthworm is better, but shed tail is pretty tough to make use of consistently, since it's going to take a hit and potentially end up below shed tail range. Failing shed tail is a real momentum drain too.
Shed Tail aside, Orthworm has been putting in work defensively. With tera ghost it completely walls the new Donphans, and hits pretty hard with Body Press too. I find it needs another attacking move so ghosts can't come in on it and set up for free though. Everyone expects it to shed tail, so I find just attacking or spiking gets a lot done while they waste time trying to stop Shed Tail.
Breloom is incredible. Mach Punch is obviously great with all the darks running around, and loaded dice bullet seed destroys things that try to switch in on it. I've found Spore is only really helpful against slow walls to get a free switch, since if I predict a switch to take Mach Punch I'd much rather just Bullet Seed. Scizor is worse but being able to take physical hits is nice and Bullet Punching Flutter Mane, Chien Pao and Iron Valiant is great.
Glimmora was great at first, but people are increasingly running things like Corviknight and Clodsire to take hits, which make it kind of a wasted slot. I haven't seen any of those Pokemon have trouble with the recovery move nerf like everyone was expecting pre-release, but I also haven't seen any stall, just bulkier offense. Against teams that can't remove Toxic Spikes it's still great though.
Scarf Chi Yu just kind of obliterates teams, and can take a decent number of moves too. Priority can handle it, but it might be too much of a teambuilding constraint to need so much priority, especially if Palafin gets banned.
Chien Pao seems strong, but not on par with the two big threats. Iron Bundle as well, though tera water Iron Bundle makes it a lot better. Houndstone seems like it comes down to how well the other team can deal with the pokemon that can deal with it before late game, so as people optimize building around it I think it might become too much, but it doesn't seem like it can just get slapped on any team and work. Saving terastalizing until the end of the game for it is also really good, since they probably don't have multiple answers by that point.
Terastalizing is fine right up to the point where offensive threats dodge a check by defensively terastalizing. I think if anything pushes it into banworthy status, it'll be that. That being said, it's currently hard to predict what type people will use. It might be more reasonable when things settle down and people are running more optimized choices you can expect and prepare for. After the first turn they do it you can play around the new type, and just terastalizing for more damage doesn't seem broken.
Meowscarada, Lokix, and Maushold seem terrible. I've run into them a lot and they never really do anything. Especially Meowscarada, which just kind of comes in and dies. Roaring Moon and Iron Valiant seem pretty underwhelming as well, unless they terastalize defensively to no longer fold to priority. Priority is really good in general and a lot of the hyped up new stuff can't handle it. Maybe once psychic terrain comes back some of these threats will be a lot harder to deal with, but that's a ways away.
