So some initial thoughts on a few Pokemon after using them/facing them:
Minior: Likely OU or BL, it's form changing + Shell Smash is what makes it so strong. You can easily get a Shell Smash in with 60/100/100 for HP and defenses, slap a White Herb on it so it gets those defense losses wiped, and if you're lucky the opposing player will get you to 50% and activate Shields Down. Then you can simply wipe the floor with Acrobatics, Earthquake, and Stone Edge.
Mudsdale: Likely UU, I thought it would easily be OU, but it's just 'eh'. It's ability is great, but it would enjoy SpD boosts more than Def boosts. It hits hard, but Tapu Bulu just straight up counters it hard, and Pokemon like Celesteela and Kartana don't mind it's attacks and defense boosts. I can't see it being any higher or lower than UU.
Toxapex: Torn between UU and OU. It looks AMAZING on paper, but in action it's more of a less useful and more annoying Ferrothorn. With Steel/Poison types being immune to poison and Poison types wiping toxic spikes from the field, it literally checks and counters itself, and honestly it would have good synergy with Salazzle in doubles with a Merciless set, but it's attack stats are nothing special and the two get hit HARD by Earthquakes, so unless you're running Air Balloon on Salazzle and Baneful Bunker on Toxapex it won't work out too greatly in the long run.
Mimikyu: UU or BL. It's ability allows you to get a free Bulk Up or Swords Dance, which is absolutely nothing to shrug off. This thing has max 320 speed, which is pretty decent, but it also has Shadow Sneak and 3 immunities. Slap a Life Orb on it and you'll be dealing a massive amount of damage. You can either choose full out offense set with SD or a bulkier set with Bulk Up, either works perfectly.
Celesteela: OU. This thing is scary. It's stat spread allows you to choose which stat you want to boost via Beast Boost, and that combined with it's amazing bulk and pretty good movepool makes it a force to be reckoned with. It can go special, physical, mixed, and bulky. It's got an amazing typing which only makes it weak to Fire and Electric and grants it two immunities.
Pheromosa: Honestly don't know. It's not as OP as many claim it is tbh. It literally dies to most priority moves, it's a great wallbreaker, but the fact it just boosts speed after each kill instead of Att/SpA is upsetting. However, it's like a Deoxys rip-off, so I can see it being Ubers worthy, but I think it may remain OU.
Komala: RU, it's an amazing status absorber. It'll gladly take any sleep-inducing move without a care in the world since it can still attack whilst sleeping, and it has a great movepool with some aight stats. It has access to Rapid Spin/Wish/Bulk Up/Return/Play Rough/Earthquake/Wood Hammer/Sucker Punch/U-Turn/Swords Dance/Shadow Claw/Rock Slide and more. However the fact it's so slow like most mons' this gen keeps it from being TOO good, and due to it's low defense, it won't be happy to face-off against anything with Close Combat/Superpower/Hammer Arm/Drain Punch/etc...
Still toying around with other mons', but these are my thoughts on some of the ones I've seen/tried out. Also, Porygon-Z with Z-Conversion + Shadow Ball is super scary.